Attribute inspection

[RazerM]

Would a PR be accepted that pulled out part of this code:

marshmallow-jsonapi/marshmallow_jsonapi/schema.py

Lines 320 to 341 in d3ef934

	attributes = {
	(self.fields[field].dump_to or field): field
	for field in self.fields
	}
	for field_name, value in iteritems(item):
	attribute = attributes[field_name]
	if attribute == ID:
	ret[ID] = value
	elif isinstance(self.fields[attribute], Meta):
	if 'meta' not in ret:
	ret['meta'] = self.dict_class()
	ret['meta'].update(value)
	elif isinstance(self.fields[attribute], BaseRelationship):
	if value:
	if 'relationships' not in ret:
	ret['relationships'] = self.dict_class()
	ret['relationships'][self.inflect(field_name)] = value
	else:
	if 'attributes' not in ret:
	ret['attributes'] = self.dict_class()
	ret['attributes'][self.inflect(field_name)] = value

into a separate function? Specifically, the part that creates a map of JSONAPI attributes to Marshmallow fields?

The use case I have in mind is being able to tell which attributes I'll end up with before dumping. I would still need to apply inflect(attribute) and check field_obj.load_only, but at least I don't have to rely on internal API.