Merge r1808746, r1809028 from trunk:

mod_rewrite/core: avoid the 'Vary: Host' header

In PR 58231 is was brought up that httpd adds the
Vary: Host header whenever a condition is set to true
in mod_rewrite or in an <If> block.

The https://tools.ietf.org/html/rfc7231#section-7.1.4
section seems to disallow this use case:

"The "Vary" header field in a response describes "
"what parts of a request message, "
"aside from the method, Host header field, [...]"

I had a chat with the folks in #traffic-server and
they don't see much point in having a Vary: Host header,
plus it was reported that Varnish doesn't like it very
much (namely it does not cache the response when
it sees the header, links of the report in the PR).

I don't see much value in this behavior of httpd so
I am inclined to remove this response header value,
but I'd be glad to get a more experienced opinion.



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 su...