Make javascript key optional (#570)

Close #569

Author: alvinthen

src/ParseFile.js

@@ -243,9 +243,12 @@ var DefaultController = {
     // To directly upload a File, we use a REST-style AJAX request
     var headers = {
       'X-Parse-Application-ID': CoreManager.get('APPLICATION_ID'),
-      'X-Parse-JavaScript-Key': CoreManager.get('JAVASCRIPT_KEY'),
       'Content-Type': source.type || (source.file? source.file.type : null)
     };
+    var jsKey = CoreManager.get('JAVASCRIPT_KEY');
+    if (jsKey) {
+      headers['X-Parse-JavaScript-Key'] = jsKey;
+    }
     var url = CoreManager.get('SERVER_URL');
     if (url[url.length - 1] !== '/') {
       url += '/';