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
Hello, Foobar
['Hello, Fo, =;:<o', 'Hello, Bar']