NH-3741 - Many to many mapping with interfaces.

Fix mapping bug when generic collection contains interfaces instead of entities (Jira #NH-3741)
In our normal mappings we have done.  The tests passed without, so I left it out (on the theory of keeping the tests as small as possible).  We haven't gotten the full solution that we found the bug in updated to NH4 yet, so not tested in that if we can do without the class declaration.