Fixed Bug 9576. Corrects the way System.Uri handles slashs in the path.

Makes more compliant to RFC 3986 and fixes the Bug 9576 (https://bugzilla.xamarin.com/show_bug.cgi?id=9576). 

Tests weren't changed, they need to be reviewed by someone who really understands the original purpose of the tests, since they were written specifically to assert the unencoding of slash -- despite the RFC 3986 stating otherwise. Also .NET 4.5 has fixed the same bug recently. Apparently there's a work in progress to make the System.Uri compliant to RFC 3986, under Bug 5887 (https://bugzilla.xamarin.com/show_bug.cgi?id=5887).

Patch granted under the current MIT X11 licence.