Merge pull request #579 from yang-g/c++api

change metadata getters to return const&

include/grpc++/client_context.h

@@ -81,12 +81,12 @@ class ClientContext {
   void AddMetadata(const grpc::string &meta_key,
                    const grpc::string &meta_value);
 
-  std::multimap<grpc::string, grpc::string> GetServerInitialMetadata() {
+  const std::multimap<grpc::string, grpc::string>& GetServerInitialMetadata() {
     GPR_ASSERT(initial_metadata_received_);
     return recv_initial_metadata_;
   }
 
-  std::multimap<grpc::string, grpc::string> GetServerTrailingMetadata() {
+  const std::multimap<grpc::string, grpc::string>& GetServerTrailingMetadata() {
     // TODO(yangg) check finished
     return trailing_metadata_;
   }

include/grpc++/server_context.h

@@ -76,7 +76,7 @@ class ServerContext final {
   void AddInitialMetadata(const grpc::string& key, const grpc::string& value);
   void AddTrailingMetadata(const grpc::string& key, const grpc::string& value);
 
-  std::multimap<grpc::string, grpc::string> client_metadata() {
+  const std::multimap<grpc::string, grpc::string>& client_metadata() {
     return client_metadata_;
   }