Adding content-header to api endpoints

[dalanmiller]

What does this PR do?

This PR (re-?)adds Content-Type to the Traefik /api endpoints.

I'm unsure if I'm meeting the testing standards but I modified as little as I thought was necessary. Completely happy to receive feedback as I'm not primarily a Golang developer.

Motivation

I noticed in the recent alpha7 that the raw JSON being returned was not being formatted nicely in Firefox anymore. This was working in the past, but I didn't notice any changes to this file which would've added this in the history. I may be unaware of the appropriate Headers being added in some other http middleware.

Example response headers from alpha7 /api/rawdata

HTTP/1.1 200 OK
X-Next-Page: 1
Date: Tue, 25 Jun 2019 13:30:58 GMT
Content-Length: 1338
Content-Type: text/plain; charset=utf-8

More

• Added/updated tests
• Added/updated documentation

Additional Notes

[ldez]

Thanks 👍

[dtomcej]

LGTM