Incorrect NS prefix for XSLT namespace

When non-conventional namespace prefixes are used for
standard namespaces (for example, if the prefix "xslt"
is used for the XSLT namespace, rather than the
conventional "xsl") Saxon may serialize such names in
the output file using the conventional prefix, while
generating a namespace declaration for the prefix
actually used in the source.

Source code fixed (in NamePool#getDisplayName); test
case added (nspc54).