Fix error when validating new member in CP (#40)

Author: snavebelac

src/Http/Controllers/CP/MembersController.php

@@ -125,7 +125,7 @@ public function store(Request $request)
 
         $fields = $blueprint->fields()->only($only)->except($except)->addValues($request->all());
 
-        $fields->validate(['email' => 'required|email|unique_user_value']);
+        $fields->validate(['email' => ['required', 'email', new UniqueUserValue()]]);
 
         $values = $fields->process()->values()->except(['email', 'groups', 'roles', 'password']);