Wrong content type when "Accept: application/json;charset=utf-8" is specified

Compare responses to
```bash
$ curl -I -H "Accept: application/json" http://hackage.haskell.org/users/
```
and
```bash
$ curl -I -H "Accept: application/json;charset=utf-8" http://hackage.haskell.org/users/
```

First one has `Content-Type: application/json; charset=utf-8` as expected, but the second one has `Content-Type: text/html; charset=utf-8` (and the response body would also be html in that case).