Make sure xhr req header's value is String type

Summary:
In the `NetworkingModule.java`, `header.getString(1)` was
called, so the value must be String type.

FIX #10198
Closes https://github.com/facebook/react-native/pull/10222

Differential Revision: D4080319

Pulled By: lacker

fbshipit-source-id: 85234a2bbf90e5b9e0e65ceadbfabb330b2d1322

Libraries/Network/XMLHttpRequest.js

@@ -352,7 +352,7 @@ class XMLHttpRequest extends EventTarget(...XHR_EVENTS) {
     if (this.readyState !== this.OPENED) {
       throw new Error('Request has not been opened');
     }
-    this._headers[header.toLowerCase()] = value;
+    this._headers[header.toLowerCase()] = String(value);
   }
 
   /**