Write a new request specs to test Workhorse (etag) caching

The goal is to write new request specs to test that Workhorse caching works (using Etag) as expected. See gitlab-workhorse!270 (comment 84793971).

Quoting @jacobvosmaer-gitlab:

For instance we could make the same request two times in a row. Can we reliably see, perhaps in the logs, that the second request didn't reach gitlab-rails?

This could just be new request specs since we're actually using Workhorse in the gitlab-rails test suite:

For every route at https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/etag_caching/router.rb:

1. Makes a first request
2. Checks that the request returns a 200
3. Makes the same request again
4. Checks that the request returns a 304