tests: use $HTTP_DATE$ for Date header
Support for the $HTTP_DATE$ substitution was recently added. Let's
adopt it more widely.
We had to tweak the substitution to be case insensitive, since
HTTP headers are case insensitive. I also found a minor test
issue not globbing over the length of the Server response header.
Differential Revision: https://phab.mercurial-scm.org/D2839
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors