Fixes #230. Keep write only fields from having an attribute key

Author: jerel

rest_framework_json_api/renderers.py

@@ -43,6 +43,9 @@ def extract_attributes(fields, resource):
             # ID is always provided in the root of JSON API so remove it from attributes
             if field_name == 'id':
                 continue
+            # don't output a key for write only fields
+            if fields[field_name].write_only:
+                continue
             # Skip fields with relations
             if isinstance(field, (relations.RelatedField, relations.ManyRelatedField, BaseSerializer)):
                 continue