Emit warnings after off is called (#1513)

* Emit warnings after off is called

If a handler has been registered, and then gets unregistered, the key/methodname still exists in the map and signalr does not warn about a missing handler. Might be by design, but I like it this way.

client-ts/signalr/src/HubConnection.ts

@@ -267,6 +267,9 @@ export class HubConnection {
         const removeIdx = handlers.indexOf(method);
         if (removeIdx !== -1) {
             handlers.splice(removeIdx, 1);
+            if (handlers.length === 0) {
+                this.methods.delete(methodName);
+            }
         }
     }