[xml] should xmlAddChild update the properties doc ptr?

["Vakoc, Mark" <Mark_Vakoc jdedwards com>]

When moving nodes from one document to another using first xmlUnlinkNode()
then xmlAddChild() to a different document, the node added to the target
document does not update the properties' doc ptr.  xmlAddChild() calls
xmlSetTree() which correctly modifes all the child nodes to have the correct
doc ptr, but all the attributes (properties) are not updated.

I haven't encountered any problems caused by this yet, but is this the
correct behavior (or, more bluntly, is an an element node's
cur->properties->doc ever used?)