[DDC-3170] SimpleObjectHydrator fails to get discriminator column from mapped SQL result

This PR fixes [DDC-3170](http://www.doctrine-project.org/jira/browse/DDC-3170).

When querying a simple entity which uses single table- or class table inheritance using simple object hydration (``AbstractQuery::HYDRATE_SIMPLEOBJECT``), the mapped discriminator column was not retrieved correctly.

If the column got an alias during result set mapping other than it's actual name (e.g. ``type34`` instead of ``type``) than this alias wasn't correctly resolved when retrieving the discriminator column from the SQL result set.
@ureimers can this operation's complexity be reduced? I know discriminator values are usually in a small range, but the function call and the lookup within this loop seem quite heavy to me.