optional 400 response

[gsemet]

Please do not add the 400 response automatically in all api, some cannot fail.

Thanks

def get_swagger_operation(self, context=default_context):
        """
        get the swagger_schema operation representation.
        """
        consumes = produces = context.contenttype_serializers.keys()
        parameters = get_swagger_parameters(self.parameters, context)
        responses = {
            "400": Response({
                "description": "invalid input received",