[GH-237] Do not treat assignment operator as a parameter DBAL-398

This issue is created automatically through a Github pull request on behalf of roverwolf:

Url: https://github.com/doctrine/dbal/pull/237

Message:

A native (mysql) query should be able to use the assignment operator := but currently the SQLParserUtils method treats that as a missing parameter.

The patch solves the issue, but it looks like a higher-level fix might be desired that also handles the regular expression better.