Set correct Content-Type header in query response

[chaudum]

Because the internal query request handlers did not set the response
content type explicitly to application/json, and the external request
handler takes the response content type from the internal response to
set it for the resulting HTTP response, that response was also
incorrectly returning Content-Type: text/plain.

Setting the content type in the internal response correctly results in
the expected Content-Type header application/json; charset=UTF-8.

Fixes #4791

Signed-off-by: Christian Haudum christian.haudum@gmail.com

$ curl -vGET http://localhost:3100/loki/api/v1/query_range --data-urlencode 'query={app="app"}' 
*   Trying 127.0.0.1:3100...
* Connected to localhost (127.0.0.1) port 3100 (#0)
> GET /loki/api/v1/query_range?query=%7Bapp%3D%22app%22%7D HTTP/1.1
> Host: localhost:3100
> User-Agent: curl/7.79.1
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Content-Type: application/json; charset=UTF-8
< Date: Thu, 25 Nov 2021 21:15:13 GMT
< Content-Length: 735
< 

[cyriltovena]

LGTM