Fix incorrect lowercase response headers set for XHR responses

Summary:
Trivial change to fix the lowercase response headers set for XHR responses.

What would happen is the first iterated header wouldn't be part of `_lowerCaseResponseHeaders`.
Also it would mutate the original `responseHeaders` object, mixing lowercase headers with the original values.
Closes https://github.com/facebook/react-native/pull/1876
Github Author: Jean Regisser <jean.regisser@gmail.com>

Libraries/Network/XMLHttpRequestBase.js

@@ -154,7 +154,7 @@ class XMLHttpRequestBase {
     this._lowerCaseResponseHeaders =
       Object.keys(headers).reduce((lcaseHeaders, headerName) => {
         lcaseHeaders[headerName.toLowerCase()] = headers[headerName];
-        return headers;
+        return lcaseHeaders;
       }, {});
   }