BUG/MAJOR: htx/backend: Make all tests on HTTP messages compatible with HTX

BUG/MAJOR: htx/backend: Make all tests on HTTP messages compatible with HTX

A piece of code about the HTX was lost this summer, after the "big merge"
(htx/http2/connection layer refactoring). I forgot to keep HTX changes in the
functions connect_server() and assign_server(). So, this patch fixes "uri",
"url_param" and "hdr" LB algorithms when the HTX is enabled. It also fixes
evaluation of the "sni" expression on server lines.

This issue was reported on github. See issue #32.

This patch must be backported in 1.9.