Update MySqlConnector.php to support Sphinx

Sphinx is one of the 4 main search engines PHP has documented and provides classes for.

http://www.php.net/manual/en/refs.search.php

Sphinx uses the mysql protocol as an interface.  The problem is when passing collate the connection to sphinx fails.  The patch will not include collate in the connection if in the config file its set to null.

If accepted please let me know if I should make the same pull request for 4.0.x

Thank you