mod_cluster DefaultMCMPHandler should handle "Connection: close" response header and close a connection

mod_cluster DefaultMCMPHandler#sendRequest() does not close a connection when Apache httpd closes a connection and responds to MCMP STATUS request with "Connection: close" response header. (As KeepAlive Off is set by default, Apache httpd closes a connection and responds to MCMP STATUS request with "Connection: close".) Therefore, CLOSE_WAIT connection remains every MCMP STATUS command.

This CLOSE_WAIT connection will be cleaned when trying to send the next MCMP STATUS request, so there's no functional impact. And this is not a critical issue as neither connection leak nor file descriptor leak happens. However, it's better that mod_cluster handles the "Connection: close" response header and close a connection.