Use correct write in Backend::execute_session_command

Backend::execute_session_command would use the overridden write method
instead of the Backend::write method that it intended to use. This caused
session commands that did not expect a response to be in a state that
expected a result.

Also fixed RWBackend::write pass the response_type value to
Backend::write.

server/modules/routing/readwritesplit/rwsplitsession.cc

@@ -96,7 +96,7 @@ bool RWBackend::write(GWBUF* buffer, response_type type)
         }
     }
 
-    return mxs::Backend::write(buffer);
+    return mxs::Backend::write(buffer, type);
 }
 
 bool RWBackend::consume_fetched_rows(GWBUF* buffer)