Fix relative redirect handling in API
Fix relative redirect handling in API

The Location header can be a relative path from the requested URL, and
so should be resolved to a full URL.

Poison helpfully just crashes the process with a `badarg` if you don't do
this, so Phoenix gives you no stacktrace. It's great.

wfleming/fourohfourfinder,wfleming/fourohfourfinder,wfleming/fourohfourfinder