[GH-210] MappingDriverChain::getAllClassNames should load all classes from the defaultDriver

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

Url: https://github.com/doctrine/common/pull/210

Message:

I actually tried working around this by adding a driver using an empty string as the namespace, only to find out that `strpos()` doesn't accept an empty delimiter.

Anyway, this makes sure that all loadable classes for the defaultDriver are actually returned by MappingDriverChain as well.