remove invalid resource types from the metadata

We have seen several mlses list a Resource type in the metadata, but
then never reference in the list of classes. This breaks our
metadata, as we assumed all resources had a class. This fixes that
potential bug.
Merge remote-tracking branch 'origin/master' into handle_missing_rets_resources
Metadata can be created via deserialization, constructor is not called
Merge pull request #76 from estately/handle_missing_rets_resources

remove invalid resource types from the metadata