Add idempotent test

src/objective-c/tests/GRPCClientTests.m

@@ -317,4 +317,37 @@ static GRPCProtoMethod *kUnaryCallMethod;
 
 }
 
+- (void)testIdempotentProtoRPC {
+  __weak XCTestExpectation *response = [self expectationWithDescription:@"Expected response."];
+  __weak XCTestExpectation *completion = [self expectationWithDescription:@"RPC completed."];
+
+  RMTSimpleRequest *request = [RMTSimpleRequest message];
+  request.responseSize = 100;
+  request.fillUsername = YES;
+  request.fillOauthScope = YES;
+  GRXWriter *requestsWriter = [GRXWriter writerWithValue:[request data]];
+
+  GRPCCall *call = [[GRPCCall alloc] initWithHost:kHostAddress
+                                             path:kUnaryCallMethod.HTTPPath
+                                   requestsWriter:requestsWriter];
+  [GRPCCall setCallAttribute:GRPCCallAttrIdempotentRequest host:kHostAddress path:kUnaryCallMethod.HTTPPath];
+
+  id<GRXWriteable> responsesWriteable = [[GRXWriteable alloc] initWithValueHandler:^(NSData *value) {
+    XCTAssertNotNil(value, @"nil value received as response.");
+    XCTAssertGreaterThan(value.length, 0, @"Empty response received.");
+    RMTSimpleResponse *responseProto = [RMTSimpleResponse parseFromData:value error:NULL];
+    // We expect empty strings, not nil:
+    XCTAssertNotNil(responseProto.username, @"Response's username is nil.");
+    XCTAssertNotNil(responseProto.oauthScope, @"Response's OAuth scope is nil.");
+    [response fulfill];
+  } completionHandler:^(NSError *errorOrNil) {
+    XCTAssertNil(errorOrNil, @"Finished with unexpected error: %@", errorOrNil);
+    [completion fulfill];
+  }];
+
+  [call startWithWriteable:responsesWriteable];
+
+  [self waitForExpectationsWithTimeout:8 handler:nil];
+}
+
 @end