Makes GRPCRequestHeaders a NSMutableDictionary

TODO:
- Documentation
- Make public
- Check I’ve implemented all NSMutDict required methods
diff --git a/src/objective-c/GRPCClient/GRPCCall.m b/src/objective-c/GRPCClient/GRPCCall.m
index b6986bf..f79b7d0 100644
--- a/src/objective-c/GRPCClient/GRPCCall.m
+++ b/src/objective-c/GRPCClient/GRPCCall.m
@@ -221,7 +221,7 @@
 
 #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]]];