Add proxy layer to calculate content type and disposition headers

https://gitlab.com/gitlab-org/gitlab-ce/issues/36103

https://dev.gitlab.org/gitlab/gitlab-workhorse/merge_requests/3

This MR introduces a new wrapper for some 'explicitly configured' Workhorse requests. It buffers the request until it has enough data to calculate the proper content type and disposition.

For now, this behavior is experimental. Because of that, we're hiding it behind a feature flag. If the header Gitlab-Workhorse-Detect-Content-Type is present in the response, the new functionality will run.