Set the content type as application/json

I copied off the jsonify function like you told me to check out from 
https://github.com/UWFlow/rmc/blob/master/server/api/api_util.py#L91
I removed the ApiJsonEncoder, wasn't sure if it was needed here. Let me know if I should add it back.
nit: Prefer formatting like this:

```python
return api_util.jsonify({
    'tags': plugin['tags']
})
```