The DoH work (#1619) made changes to pkg/nonwriter.Writer that in
hindsight were not backwards compatible; it added override for the
LocalAddr() and RemoteAddr(). Instead of rolling back that PR, this PR
reverts those changes and creates a DoHWriter for use in the
https-server.go side of things.

This was only caught in the integration test making this hard to catch,
so we add a upstream_file_test.go that tries (doesn't work yet) to test
this in the unit tests as well. Esp. helpful when 'git bisecting'.

Fixes #1826