Split off #270

The most recent version broke support for default DRF serializers for reverse and m2m relations (the wrong model type gets returned).

Although the recommended way is to use serializers supplied by jsonapi, we should try to maintain support for drf serializers so long as it's not too big of a job to do so.

• Add failing tests (using from rest_framework import serializers vs. from restframework_json_api import serializers
• Fix breaking change (probably a bug in utils.get_related_resource_type)