Resolve "Gracefully handle missing Content-Length Header from API"

What does this MR do?

This MR resolves #1078 (closed). It ensures that Gitlab Pages does not include a Content-Length header if it did not receive one from the API.

By default, go's http client will handle a missing Content-Length header by using -1 as the value. If we pass this on to the client, we're actually using a value that's invalid as per the HTTP spec (see the issue for details)

This MR introduces a check whether or not the Content-Length value is valid, and only if this is true, adds a Content-Length header to its own response.

TODO

• Feature flag
  • Added feature flag:
  • This feature does not require a feature flag
• I added the Changelog trailer to the commits that need to be included in the changelog (e.g. Changelog: added)
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines

Closes #1078 (closed)