Preserve caller expectations for behaviour of sslVerifyHost

(bug 42441) The previous patch unnecessarily broke backwards
compatibility in the Http::request() API, following cURL's broken
conventions for sslVerifyHost instead of the boolean interpretation
expected by all existing callers. This change reverts that one, and
fixes the bug in another way. See Ia6535f10.

Also don't bother wrapping the $this->sslVerifyHost access with isset()
since it's always set.

Change-Id: Ia4e1689249b6ac515b859ea2eca1dcff3e63098c