Fix problem with cache key generation if the username contained an @ sign or any other special character

This fixes issue #504

src/Services/Cache/UserCacheKeyGenerator.php

@@ -60,7 +60,7 @@ class UserCacheKeyGenerator
             return 'user$_'.User::ID_ANONYMOUS;
         }
 
-        //In the most cases we can just use the username (its unique)
-        return 'user_'.$user->getUsername().'_'.$locale;
+        //Use the unique user id and the locale to generate the key
+        return 'user_'.$user->getID().'_'.$locale;
     }
 }