Restore generic relation support

Tests?

Author: AstraLuma

rest_framework_json_api/utils.py

@@ -242,6 +242,15 @@ def get_related_resource_type(relation):
 relation_model=parent_model_relation.related.related_model
 else:
 relation_model=parent_model_relation.related.model
+elifparent_model_relation_typeisManyToManyDescriptor:
+relation_model=parent_model_relation.field.remote_field.model
+elifparent_model_relation_typeisReverseManyRelatedObjectsDescriptor:
+relation_model=parent_model_relation.field.related.model
+elifparent_model_relation_typeisReverseGenericManyToOneDescriptor:
+ifdjango.VERSION>= (1, 9):
+relation_model=parent_model_relation.rel.model
+else:
+relation_model=parent_model_relation.field.related_model
 elifhasattr(parent_model_relation, 'field'):
 try:
 relation_model=parent_model_relation.field.remote_field.model