patch the previous fix

src/objective-c/GRPCClient/GRPCCallOptions.h

@@ -226,7 +226,7 @@ typedef NS_ENUM(NSInteger, GRPCTransportType) {
 /**
  * Initial metadata key-value pairs that should be included in the request.
  */
-@property(copy, readonly) NSDictionary *initialMetadata;
+@property(copy, readwrite) NSDictionary *initialMetadata;
 
 // Channel parameters; take into account of channel signature.