* Avoid adding Content-Type to non-body responses
The current code sets the content-type header for all responses
to the result's content_type property if upstream does not set a
content_type. The default value for content_type is
"application/octet-stream".
For responses that do not have a body (like 204 No Content or
304 Not Modified), setting a content-type header is unnecessary and
potentially misleading. Follow HTTP standards by only adding the
content-type header to responses that actually contain a body.
* Add pytest for ingress proxy
* Preserve Content-Type header for HEAD requests in ingress API