Summary:
NSMutableURLRequest appears to be running `[headerValue count]` on every value of the NSDictionary that `allHTTPHeaderFields` is set to, meaning that any `null` values passed to headers in an XHR throw an exception. This differs from the implementation of XHR in browsers, which just ignore any `null` values for headers.
PR doesn't currently include tests, happy to add those if this is something you'd consider merging
Closes https://github.com/facebook/react-native/pull/6903
Differential Revision: D3256727
fb-gh-sync-id: 58e8db9c86a1fe7fbd95cba7dcf916957d179e0c
fbshipit-source-id: 58e8db9c86a1fe7fbd95cba7dcf916957d179e0c