Merge branch '76-only-one-content-type-header-in-archive' into 'master'

Avoid setting multiple values for certain headers

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

See merge request !84