Update core/model/modx/sources/modfilemediasource.class.php

See http://tracker.modx.com/issues/8392

The base_url should not be prefixed when you setup a media source relative
Do not prepend base_url when baseUrlRelative in modFileMediaSource

Fixes http://tracker.modx.com/issues/8392
Merges #358

Merge remote-tracking branch 'origin/pr/358' into develop

* origin/pr/358:
  Update core/model/modx/sources/modfilemediasource.class.php