custom validation func being called on view PATCH.

As per the instructions in Issue #965, this is a failing test that shows a serializer's custom validation being called for a non-required field on a PATCH call to a view. 

On line 1703, remove the trailing comment to have the test pass. So line 1703 changes form ```partial_data = {'title': self.right_title}``` to ```partial_data = {'title': self.right_title, 'artist': self.band.id}```.