Set content type to "application/json" for JSON responses

Deployed to staging:

```
curl -i http://staging.api.lobid.org/oer/1f4269a9-0103-4d9f-913c-5f127a5dab88
HTTP/1.1 200 OK
Date: Thu, 27 Mar 2014 15:00:28 GMT
Content-Type: application/json; charset=utf-8
Content-Length: 1808
```