MXS-2525: Fix non-plugin authentication

Older clients assume the plugin used for authentication is
mysql_native_password. If the client doesn't request plugin
authentication, don't treat it as an error.

server/modules/authenticator/MySQLAuth/mysql_auth.c

@@ -612,6 +612,10 @@ mysql_auth_set_client_data(
                             }
                         }
                     }
+                    else
+                    {
+                        client_data->correct_authenticator = true;
+                    }
                 }
             }
         }