Fix server crash on invalid LiveQuery socket event (#4533)

* add failing test for missing command message

* don't mock tv4.validate

* fix existing tests

* make request.op required

* Update comment

src/LiveQuery/RequestSchema.js

@@ -7,6 +7,7 @@ const general = {
       'enum': ['connect', 'subscribe', 'unsubscribe', 'update']
     },
   },
+  'required': ['op']
 };
 
 const connect =  {