When using `add_header` nginx will literally add a header. If a
`content-type` header is already configured (for example through a
server wide default), this means we end up with 2 content-type headers,
like so:

```
content-type: text/html
content-type: application/json
access-control-allow-origin: *
```

That doesn't make sense. Instead, we want the content type of that
block to only be `application/json` which we can achieve using
`default_type` instead.

Signed-off-by: Daniele Sluijters <daenney@users.noreply.github.com>