core: Fix comet/WS when site streamhost is undefined and redirect = false

In these cases, the value of the streamhost would be the site's
hostname, which causes socket / comet streams to fail when the site is
being served on another hostname (which is the case when the site config
{redirect, false} is in effect).

For this special case, this fix now sets the streamhost to the hostname
of the request.

cc @mmzeeman

(cherry picked from commit 8ffeea9e0bcb606f7a2a4d8a595cd91982dff11a)