http api should respond with correct content-type

```
> GET /api/v1/go/http_api/xxxxxxxx/messages.json HTTP/1.1
> Authorization: Basic xxxxxxxxxxxxxxxxxxxxxxxx
> User-Agent: curl/7.24.0 (x86_64-apple-darwin12.0) libcurl/7.24.0 OpenSSL/0.9.8r zlib/1.2.5
> Host: go.vumi.org
> Accept: */*
> Connection: keep-alive
>
< HTTP/1.1 200 OK
< Server: nginx/0.7.65
< Date: Mon, 01 Apr 2013 08:57:39 GMT
< Content-Type: text/html
< Transfer-Encoding: chunked
< Connection: close
<
```

The `Content-Type: text/html` in the response should in this case be `Content-Type: application/javascript; charset=utf-8`