Merge pull request #2488 from jcanizales/subclass-grxwriter

src/compiler/objective_c_generator.cc

src/objective-c/GRPCClient/GRPCCall.h

src/objective-c/GRPCClient/GRPCCall.m

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

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

src/objective-c/ProtoRPC/ProtoRPC.h

src/objective-c/ProtoRPC/ProtoRPC.m

src/objective-c/ProtoRPC/ProtoService.h

src/objective-c/ProtoRPC/ProtoService.m

src/objective-c/RxLibrary/GRXBufferedPipe.h

src/objective-c/RxLibrary/GRXForwardingWriter.h

src/objective-c/RxLibrary/GRXForwardingWriter.m

src/objective-c/RxLibrary/GRXImmediateWriter.h

src/objective-c/RxLibrary/GRXImmediateWriter.m

src/objective-c/RxLibrary/GRXWriter+Immediate.m

src/objective-c/RxLibrary/GRXWriter.h

src/objective-c/RxLibrary/GRXWriter.m

src/objective-c/RxLibrary/transformations/GRXMappingWriter.h

src/objective-c/RxLibrary/transformations/GRXMappingWriter.m

src/objective-c/tests/GRPCClientTests.m