Modify the order of postprocessor.

Currently, I do not need ```primary_key``` in json, the original logic has some problems.

Original logic:
Executive ```postprocessor``` -> Assignment ```headers```
Optimized logic:
Assignment ```headers``` -> Execute ```postprocessor```

Like:
https://github.com/jfinkels/flask-restless/blob/master/flask_restless/views.py#L964