XMLHttpRequest: ignore case for request headers

Summary:
HTTP headers are case-insensitive, so we should treat them that way when they're being set on `XMLHttpRequest`.
Closes https://github.com/facebook/react-native/pull/1381
Github Author: Philipp von Weitershausen <philikon@fb.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/Network/XMLHttpRequestBase.js

@@ -73,7 +73,7 @@ class XMLHttpRequestBase {
   }
 
   setRequestHeader(header: string, value: any): void {
-    this._headers[header] = value;
+    this._headers[header.toLowerCase()] = value;
   }
 
   open(method: string, url: string, async: ?boolean): void {