[PATCH] msxml3: Add support for loading document from relative URL provided by hosting site.

[Nikolay Sivov]

Please add some tests for that, splitting by host type too (assuming you
only need HTML document as a host judging by other patches in that bunch).

We already have similar tests for HTTPRequest object, and a helper to
get base URL for it. Maybe something could be reused. Also this is different
from httprequest case - it queries base url on every load, instead of
once in SetSite(), was that part verified somehow?