Fix failing mono tests

I noticed two tests were failing in mono

PUrify.Tests.PurifyTest+ThePurifyMethod.CanHandleAbsoluteOrRelativeUri_RelativeGiven [FAIL]
PUrify.Tests.PurifyTest+ThePurifyMethod.CanHandleRelativeUri [FAIL]

Because inside the UriInfo we access the `.Host` property, This throws on mono but works fine in .NET.

I fixed this by making `.Purify()` on non absolute uri's (those explictly marked with UriKind.Relative) a noop.