Fix updating racks collection on resource class

There were problems when some racks should be removed from the
resource class [1]. Now this is fixed and tests for this case are
added.

[1] https://github.com/tuskar/tuskar/issues/37
Remove stale fixme comment

The test now passes, so the comment can be removed.
Merge pull request #47 from jistr/ftr/racks_collection_update_fix

Resource class update fix (rack collection)