Make request headers a @property property.

src/objective-c/GRPCClient/GRPCCall.h

@@ -169,10 +169,8 @@ extern id const kGRPCTrailersKey;
 //
 // After the call is started, trying to modify this property is an error.
 //
-// For convenience, the property is initialized to an empty NSMutableDictionary, and the setter
-// accepts (and copies) both mutable and immutable dictionaries.
-- (id<GRPCRequestHeaders>)requestHeaders; // nonatomic
-- (void)setRequestHeaders:(NSDictionary *)requestHeaders; // nonatomic, copy
+// The property is initialized to an empty NSMutableDictionary.
+@property(atomic, readonly) id<GRPCRequestHeaders> requestHeaders;
 
 // This dictionary is populated with the HTTP headers received from the server. This happens before
 // any response message is received from the server. It has the same structure as the request

src/objective-c/GRPCClient/GRPCCall.m

@@ -130,20 +130,6 @@ NSString * const kGRPCTrailersKey = @"io.grpc.TrailersKey";
   return self;
 }
 
-#pragma mark Metadata
-
-- (id<GRPCRequestHeaders>)requestHeaders {
-  return _requestHeaders;
-}
-
-- (void)setRequestHeaders:(NSDictionary *)requestHeaders {
-  GRPCRequestHeaders *newHeaders = [[GRPCRequestHeaders alloc] initWithCall:self];
-  for (id key in requestHeaders) {
-    newHeaders[key] = requestHeaders[key];
-  }
-  _requestHeaders = newHeaders;
-}
-
 #pragma mark Finish
 
 - (void)finishWithError:(NSError *)errorOrNil {