Fix New-Object for System.Uri with UriKind parameter.

Running the following command:

    New-Object Uri('http://example.org', [System.UriKind]::Absolute)

Would fail with the error:

    Constructor on type 'System.Uri' not found.

The problem was that the UriKind argument was a PSObject and needed
to be unwrapped before it was passed to Activator.CreateInstance(...)