Disabled duplicate "Host" headers (ticket #1724).

Duplicate "Host" headers were allowed in nginx 0.7.0 (revision b9de93d804ea)
as a workaround for some broken Motorola phones which used to generate
requests with two "Host" headers[1].  It is believed that this workaround
is no longer relevant.

[1] http://mailman.nginx.org/pipermail/nginx-ru/2008-May/017845.html