Parse HTTP target starting with //

[gregw]

The HTTP specification rfc7230 5.3 says that a request line has a target that is either:

request-target = origin-form
                    / absolute-form
                    / authority-form
                    / asterisk-form
origin-form    = absolute-path [ "?" query ]

so essentially it is either an absolute URI, a URI path, an authority or an asterisk.

However, HttpURI class treats a URI like //foo/bar as host=foo path=/bar, which is correct for URIs generally not not specifically for a HTTP target.