Fixed the many to many behavior.

There's an issue with nested writable serializers when the models are linked by a many to many field.
At this point I think the nested writable serializers are too complex to be maintainable without a major re-think.
This line is a good example of that - I can't at all easily determine why it would be needed or what cases it would effect.  I've been spending some time considering how to address all this properly, and I'm hoping to address a redesign shortly.