Allow uploading a native file (e.g. photo) via XMLHttpRequest

Summary:
With this in place, it's possible to upload a picture from the `CameraRoll` to Parse, for instance:

    xhr = new XMLHttpRequest();
    xhr.onload = function() {
      data = JSON.parse(xhr.responseText);
      var parseFile = new Parse.File(data.name);
      parseFile._url = data.url;
      callback(parseFile);
    };
    xhr.setRequestHeader('X-Parse-Application-Id', appID);
    xhr.setRequestHeader('X-Parse-JavaScript-Key', appKey);
    xhr.open('POST', 'https://api.parse.com/1/files/image.jpg');
    // assetURI as provided e.g. by the CameraRoll API
    xhr.send(new NativeFile(assetURI));

Closes https://github.com/facebook/react-native/pull/1357
Github Author: Philipp von Weitershausen <philikon@fb.com>

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

Libraries/Network/XMLHttpRequest.ios.js

@@ -20,13 +20,16 @@ var XMLHttpRequestBase = require('XMLHttpRequestBase');
 class XMLHttpRequest extends XMLHttpRequestBase {
 
   sendImpl(method: ?string, url: ?string, headers: Object, data: any): void {
+    if (typeof data === 'string') {
+      data = {string: data};
+    }
     RCTDataManager.queryData(
       'http',
       {
-        method: method,
-        url: url,
-        data: data,
-        headers: headers,
+        method,
+        url,
+        data,
+        headers,
       },
       // TODO: Do we need this? is it used anywhere?
       'h' + crc32(method + '|' + url + '|' + data),