Make the client passed to HtCat actually be used

An oversight.
Deprecate use of HTTP HEAD: instead using GET piggy-backing

The general approach is to run an non-Range GET to both determine the
Content-Length and also to start downloading bytes in one shot, rather
than waiting for a response to HEAD before beginning any bulk data
transfer.

This both eliminates some startup time that can compound when dealing
with smaller GETs and can avoid failure when working against a server
that does not support HTTP HEAD in a situation where htcat would
otherwise work fine.