Revision 1809028

mod_rewrite,core: avoid Vary:Host (part 2)

This is a follow up of r1808746 after a chat
with Yann on dev@:

- the HTTP:Host variable suffers from the same problem
- the strcasecmp should be used to allow case-sensitive
  comparisons.
- in mod_rewrite is less cumbersome and more clean to just
  make the Host header check in lookup_header, so it will
  be automatically picked up by every part of the code
  that uses it. It shouldn't be a relevant overhead for
  mod_rewrite.