[IRI] Changed preferred case when hex-escaping IRIs

Based on deliberations by the TAG
(http://www.w3.org/2003/01/20-tag-summary), I have changed the
preferred case when hex-escaping (in the process of going from
an IRI to an URI) from lower case to UPPER CASE in the current
internal draft. I have also tweaked examples where necessary.

Any comments?       Martin.