Fixed retain cycle in RCTFBHTTPRequestHandler

Summary: Following common ObjC pattern, we have to store delegates as weak pointers.

Reviewed By: mmmulani

Differential Revision: D6558886

fbshipit-source-id: 11a710a7e9e17d7c6a78ae46b53b043f44ccf8e5

React/Base/RCTURLRequestHandler.h

@@ -29,6 +29,7 @@
  * itself. This will be used later to refer to the request in callbacks. The
  * `sendRequest:withDelegate:` method *must* return before calling any of the
  * delegate methods, or the delegate won't recognize the token.
+ * Following common Objective-C pattern, `delegate` will not be retained.
  */
 - (id)sendRequest:(NSURLRequest *)request
      withDelegate:(id<RCTURLRequestDelegate>)delegate;