Add in _background parsing

Author: keithwhor

lib/gateway.js

@@ -941,7 +941,7 @@ class Gateway extends EventEmitter {
     let errors = {};
 
     // Handle special parameters
-    let specialParams = ['_bg', '_stream', '_debug'];
+    let specialParams = ['_background', '_stream', '_debug'];
     specialParams.forEach(key => {
       if (params[key]) {
         try {