Returns the content type that should be used by default on validation errors

def invalid_content_type(self, request=None, response=None):
        """Returns the content type that should be used by default on validation errors"""
        if callable(self.invalid_outputs.content_type):
            return self.invalid_outputs.content_type(request=request, response=response)
        else:
            return self.invalid_outputs.content_type