Fixing some cases where FilterSet was called with child_media_resources

Squashed commit of the following:

commit 69548f0dc5c71d61cdac90715908722e58730580
Author: Thomas Schank <DrTom@schank.ch>
Date:   Tue May 14 15:38:46 2013 +0200

    Removed wip

commit de622a5e06708718ac71423d93cbe6f03c5a63a2
Author: Thomas Schank <DrTom@schank.ch>
Date:   Tue May 14 15:38:08 2013 +0200

    Fixing a bug and a bad query

commit 19ff1461d161b373049ece1863fb32b752173f2a
Author: Thomas Schank <DrTom@schank.ch>
Date:   Tue May 14 15:15:35 2013 +0200

    Added feature test that looks into a catalog

commit b26425f652910ce0b99d00319ee94304a54f301d
Author: Thomas Schank <DrTom@schank.ch>
Date:   Tue May 14 14:08:57 2013 +0200

    Fixing some cases where FilterSet was called with child_media_resources