Makes GRPCRequestHeaders a NSMutableDictionary

TODO:
- Documentation
- Make public
- Check I’ve implemented all NSMutDict required methods

src/objective-c/GRPCClient/GRPCCall.h

@@ -161,6 +161,9 @@ extern id const kGRPCTrailersKey;
 
 #pragma mark GRPCCall
 
+/** Represents a single gRPC remote call. */
+@interface GRPCCall : GRXWriter
+
 /**
  * The container of the request headers of an RPC conforms to this protocol, which is a subset of
  * NSMutableDictionary's interface. It will become a NSMutableDictionary later on.
@@ -170,21 +173,6 @@ extern id const kGRPCTrailersKey;
  * A header value is a NSString object (with only ASCII characters), unless the header name has the
  * suffix "-bin", in which case the value has to be a NSData object.
  */
-@protocol GRPCRequestHeaders <NSObject>
-
-@property(nonatomic, readonly) NSUInteger count;
-
-- (id)objectForKeyedSubscript:(NSString *)key;
-- (void)setObject:(id)obj forKeyedSubscript:(NSString *)key;
-
-- (void)removeAllObjects;
-- (void)removeObjectForKey:(NSString *)key;
-
-@end
-
-/** Represents a single gRPC remote call. */
-@interface GRPCCall : GRXWriter
-
 /**
  * These HTTP headers will be passed to the server as part of this call. Each HTTP header is a
  * name-value pair with string names and either string or binary values.
@@ -200,7 +188,7 @@ extern id const kGRPCTrailersKey;
  *
  * The property is initialized to an empty NSMutableDictionary.
  */
-@property(atomic, readonly) id<GRPCRequestHeaders> requestHeaders;
+@property(atomic, readonly) NSMutableDictionary *requestHeaders;
 
 /**
  * This dictionary is populated with the HTTP headers received from the server. This happens before

src/objective-c/GRPCClient/GRPCCall.m

@@ -221,7 +221,7 @@ NSString * const kGRPCTrailersKey = @"io.grpc.TrailersKey";
 
 #pragma mark Send headers
 
-- (void)sendHeaders:(id<GRPCRequestHeaders>)headers {
+- (void)sendHeaders:(NSDictionary *)headers {
   // TODO(jcanizales): Add error handlers for async failures
   [_wrappedCall startBatchWithOperations:@[[[GRPCOpSendMetadata alloc] initWithMetadata:headers
                                                                                 handler:nil]]];

src/objective-c/GRPCClient/private/GRPCRequestHeaders.h

@@ -32,21 +32,14 @@
  */
 
 #import <Foundation/Foundation.h>
-#include <grpc/grpc.h>
 
 #import "../GRPCCall.h"
 
-@interface GRPCRequestHeaders : NSObject<GRPCRequestHeaders>
-
-@property(nonatomic, readonly) NSUInteger count;
-@property(nonatomic, readonly) grpc_metadata *grpc_metadataArray;
+@interface GRPCRequestHeaders : NSMutableDictionary
 
 - (instancetype)initWithCall:(GRPCCall *)call;
 
-- (id)objectForKeyedSubscript:(NSString *)key;
-- (void)setObject:(id)obj forKeyedSubscript:(NSString *)key;
-
-- (void)removeAllObjects;
-- (void)removeObjectForKey:(NSString *)key;
+- (instancetype)initWithCall:(GRPCCall *)call
+                     storage:(NSMutableDictionary *)storage NS_DESIGNATED_INITIALIZER;
 
 @end

src/objective-c/GRPCClient/private/GRPCRequestHeaders.m

@@ -68,17 +68,44 @@ static void CheckKeyValuePairIsValid(NSString *key, id value) {
 
 @implementation GRPCRequestHeaders {
   __weak GRPCCall *_call;
+  // The NSMutableDictionary superclass doesn't hold any storage (so that people can implement their
+  // own in subclasses). As that's not the reason we're subclassing, we just delegate storage to the
+  // default NSMutableDictionary subclass returned by the cluster (e.g. __NSDictionaryM on iOS 9).
   NSMutableDictionary *_delegate;
 }
 
+- (instancetype)init {
+  return [self initWithCall:nil];
+}
+
+- (instancetype)initWithCapacity:(NSUInteger)numItems {
+  return [self init];
+}
+
+- (instancetype)initWithCoder:(NSCoder *)aDecoder {
+  return [self init];
+}
+
 - (instancetype)initWithCall:(GRPCCall *)call {
+  return [self initWithCall:call storage:[NSMutableDictionary dictionary]];
+}
+
+// Designated initializer
+- (instancetype)initWithCall:(GRPCCall *)call storage:(NSMutableDictionary *)storage {
+  // TODO(jcanizales): Throw if call or storage are nil.
   if ((self = [super init])) {
     _call = call;
-    _delegate = [NSMutableDictionary dictionary];
+    _delegate = storage;
   }
   return self;
 }
 
+- (instancetype)initWithObjects:(const id  _Nonnull __unsafe_unretained *)objects
+                        forKeys:(const id<NSCopying>  _Nonnull __unsafe_unretained *)keys
+                          count:(NSUInteger)cnt {
+  return [self init];
+}
+
 - (void)checkCallIsNotStarted {
   if (_call.state != GRXWriterStateNotStarted) {
     [NSException raise:@"Invalid modification"
@@ -86,11 +113,11 @@ static void CheckKeyValuePairIsValid(NSString *key, id value) {
   }
 }
 
-- (id)objectForKeyedSubscript:(NSString *)key {
+- (id)objectForKey:(NSString *)key {
   return _delegate[key.lowercaseString];
 }
 
-- (void)setObject:(id)obj forKeyedSubscript:(NSString *)key {
+- (void)setObject:(id)obj forKey:(NSString *)key {
   [self checkCallIsNotStarted];
   CheckIsNonNilASCII(@"Header name", key);
   key = key.lowercaseString;
@@ -103,16 +130,12 @@ static void CheckKeyValuePairIsValid(NSString *key, id value) {
   [_delegate removeObjectForKey:key.lowercaseString];
 }
 
-- (void)removeAllObjects {
-  [self checkCallIsNotStarted];
-  [_delegate removeAllObjects];
-}
-
 - (NSUInteger)count {
   return _delegate.count;
 }
 
-- (grpc_metadata *)grpc_metadataArray {
-  return _delegate.grpc_metadataArray;
+- (NSEnumerator * _Nonnull)keyEnumerator {
+  return [_delegate keyEnumerator];
 }
+
 @end

src/objective-c/GRPCClient/private/GRPCWrappedCall.h

@@ -45,7 +45,7 @@
 
 @interface GRPCOpSendMetadata : GRPCOperation
 
-- (instancetype)initWithMetadata:(GRPCRequestHeaders *)metadata
+- (instancetype)initWithMetadata:(NSDictionary *)metadata
                          handler:(void(^)())handler NS_DESIGNATED_INITIALIZER;
 
 @end

src/objective-c/GRPCClient/private/GRPCWrappedCall.m

@@ -65,7 +65,7 @@
   return [self initWithMetadata:nil handler:nil];
 }
 
-- (instancetype)initWithMetadata:(GRPCRequestHeaders *)metadata handler:(void (^)())handler {
+- (instancetype)initWithMetadata:(NSDictionary *)metadata handler:(void (^)())handler {
   if (self = [super init]) {
     _op.op = GRPC_OP_SEND_INITIAL_METADATA;
     _op.data.send_initial_metadata.count = metadata.count;