Fix attribute precedence with xsl:use-attribute-sets

Commit 05f70130 broke the precedence of attributes on literal result
elements and attributes from xsl:use-attribute-sets.

Process xsl:use-attribute-sets first. Then if any attributes were added
to the target node, use xmlSetNsProp to copy the remaining attributes,
replacing the previous values.

Thanks to Alexey Neyman for the report.