Don't send the _JavaScriptKey key when none is provided

Author: andrewimm

src/RESTController.js

@@ -161,7 +161,10 @@ const RESTController = {
     }
 
     payload._ApplicationId = CoreManager.get('APPLICATION_ID');
-    payload._JavaScriptKey = CoreManager.get('JAVASCRIPT_KEY');
+    let jsKey = CoreManager.get('JAVASCRIPT_KEY');
+    if (jsKey) {
+      payload._JavaScriptKey = jsKey;
+    }
     payload._ClientVersion = CoreManager.get('VERSION');
 
     var useMasterKey = options.useMasterKey;