BUG/MINOR: http_ext: fix if-none regression in forwardfor option

BUG/MINOR: http_ext: fix if-none regression in forwardfor option

A regression was introduced in 730b983 ("MINOR: proxy: move 'forwardfor'
option to http_ext")

Indeed, when the forwardfor if-none option is specified on the frontend
but forwardfor is not specified at all on the backend: if-none from the
frontend is ignored.

But this behavior conflicts with the historical one, if-none should only
be ignored if forwardfor is also enabled on the backend and if-none is
not set there.

It should fix GH #2187.

This should be backported in 2.8 with 730b983 ("MINOR: proxy: move
'forwardfor' option to http_ext")