Fixed mapping of discriminator column

Added fix for [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`` insteaad of ``type``) than this alias wasn't reverted when retrieving the discriminator column from the SQL result set.