Changed IRequest.GetBaseUrl() to use AbsoluteUri and PathInfo

I left the check for `HttpHandlerFactory.GetBaseUrl()` as well as the fallback to using the left part of the URL plus the `HandlerFactoryPath`.  This appears to work in ASP.NET regardless of location within the domain as well as location of SS within the project.  I verified correct behavior with self-hosting as well on Windows and with Mono.  I have not tested the behavior with XSP.