connection should raise OperationalError when socket.error.
Allow 2013 error when server closes connection.
Close socket when lost connection.
Revert "Close socket when lost connection."

This reverts commit 5c226cb659ab3de12b24ccddffa98d06f411fee9.
Merge pull request #198 from PyMySQL/server-gone-away

connection should raise OperationalError when socket.error.