Avoid setting multiple values for certain headers
requested to merge nick.thomas/gitlab-workhorse:76-only-one-content-type-header-in-archive into master
Content-Type, Content-Disposition and other headers were being given multiple values in some workhorse responses, causes problems with, e.g., archive downloads.
This MR refactors the tests somewhat, and replaces calls to Header().Add() with Header().Set() where appropriate.
Closes #76 (closed)