[BUG]: Admin API do not obey content negotiation policy

[fogfish]

API responses do not specify Content-Type

e.g.

curl -vvv localhost:4001/plugins/auth_key/issue_key -H 'Content-Type: application/json' -d '{"developer_id": "1dec6c98-4593-4e2f-8a19-8d84c84d9a19"}'

*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 4001 (#0)
> POST /plugins/auth_key/issue_key HTTP/1.1
> Host: localhost:4001
> User-Agent: curl/7.54.0
> Accept: */*
> Content-Type: application/json
> Content-Length: 56
>
* upload completely sent off: 56 out of 56 bytes
< HTTP/1.1 201 Created
< server: Cowboy
< date: Fri, 19 Apr 2019 14:10:21 GMT
< content-length: 46
< cache-control: max-age=0, private, must-revalidate
<
* Connection #0 to host localhost left intact
{"key":"02ca521e-8bb3-486c-b8f2-54025022fcd6"}

Content-Type header is missing. HTTP Spec Says https://tools.ietf.org/html/rfc2616#section-7.2.1

Any HTTP/1.1 message containing an entity-body SHOULD include a Content-Type header field defining the media type of that body.

See recommendations
https://www.programmableweb.com/news/rest-api-design-put-type-content-type/2011/11/18

[phanimahesh]

Content negotiation I believe is out of scope for now, the plugin will only respond in json no matter what is requested. However, the missing header is an issue, we should add it. I'll add a quick fix in thr morning.