Expose _filter_by_type as a method

[sloria]

A small change to allow subclasses of TypedModelManager to more easily change their queryset type.

Example use case:

class PersonQuerySet(models.QuerySet):
    pass

class PersonManager(TypedModelManager):

    def get_queryset(self):
        qs = PersonQuerySet(self.model, using=self._db)
        return self._filter_by_type(qs)

[coveralls]

Coverage increased (+0.2%) to 79.771% when pulling 4cff144 on cos-forks:filter-by-type into 1a9d00d on craigds:master.