Fixes bug which causes exception when processed model contains GenericRelation field.

This is about the same issue as described in #4 and in #20.
`GenericRelation` appears in `instance._meta.many_to_many` whereas it is actually represents a one-to-many relationship. `GenericRelation` always have `.rel.through` equal `None` and it causes `AttributeError: 'NoneType' object has no attribute '_meta'` in `autofixture.base.AutoFixtureBase#process_m2m`.
PR contains test and fix.