cc @mmn
the new getRedirectUrl() didn't work with quitter.es, either from
quitter.se or quitter.no
it also looked like the HTTPClient-based function downloaded the whole
file, instead of only the headers. (but i'm not sure)
The GNU social node has configured HTTPClient (HTTP_Request2 object) with
certain characteristics, which won't be respected when just using the PHP
built-in fetchers. This way we can have the same TLS certificate policy,
timeouts etc.
Also removed one test if get_headers() had returned an array, which it
never will since the "Location" is always a single absolute URL:
"The field value consists of a single absolute URI."
- https://tools.ietf.org/html/rfc2616#section-14.30
