PR #538: 480: Attempt to allow xs:string to be 'promoted to' xs:anyURI

Hi folks,

I’ve pushed a few changes to PR #538 in an attempt to address MK’s
comments. I’m not sure it’s right, but I’m also not sure how to improve
it, so comments or suggestions welcome.

                                        Be seeing you,
                                          norm

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Norm Tovey-Walsh
Saxonica