CHEF-4086: Use "Host:" header value to construct returned URLs


 - CHEF-4086: Use "Host:" header value to construct returned URLs

Notes:
  - Requires some Pedant changes. That is forthcoming in a PR
  - I did not include the case where Hosts: is empty (and not missing).
  - How to even test this? Pedant won't catch all the cases, so additional unit testing is required.