Fix for bug 22954 - Domain-rooted relative URI is reported as absolute

This is an alternate fix for bug 22954. The original fix was reverted (see https://github.com/mono/mono/pull/1302). My justification for still fixing this is:
-The code in master as-is gives precedence to UNIX file paths over relative web paths. This is counter-intuitive and it conflicts with the documentation. For example, in the remarks section of the documentation for System.Uri, and example relative URI is given as "/new/index.htm". However Mono treats this as an absolute UNIX file path.
-The workaround requires all applications to add logic for detecting relative vs. absolute URIs if the kind of a web URI is not known (ex. a URI from a hypermedia REST response). This means URI parsing is not encapsulated by the Uri class anymore.

The difference between the original pull request and this one is that specifying "new Uri("/abc/123", UriKind.Absolute)" will still parse the URI as an absolute UNIX file path.

This is a critical issue for our application and requires us to patch Mono ourselves. Please reconsider accepting this change.