Fix many-to-many by finding related model from descriptor

[bpartridge]

In https://github.com/farhan0581/django-admin-autocomplete-filter/blob/master/admin_auto_filters/filters.py#L53 we attempt to call .get_queryset() on the descriptor of the field. For foreign keys this works fine, but for ManyToManyDescriptor (which is present if you're referencing a ManyToManyField), Django never defines get_queryset on the class or its superclass: https://github.com/django/django/blob/master/django/db/models/fields/related_descriptors.py#L748

This is kind of an oversight and broken-duck-typing on the Django team's part, and it's trivially and harmlessly monkey-patched by doing the following somewhere, which causes the filter to work as intended.

ManyToManyDescriptor.get_queryset = lambda self: self.rel.model.objects.get_queryset()

But the correct fix would be to fall back to a call like this in the filter code itself - this PR does exactly that!

[farhan0581]

Thanks buddy for your contribution. Please review your pull request once to resolve merge conflict and please raise the pull request to dev branch.

[bpartridge]

Will do once I look at @merwok's comment!

[jaredahern]

I think we may have addressed this, both in 0.6.1 and with additional changes in pre_release.