[xml] xmlTextConcat and comment nodes

[Rob Richards <rrichards ctindustries net>]

Should xmlTextConcat be able to be used with comment nodes or is it 
really necessary to handle these manually? Currently it is limited to 
text and cdata, although I see no reason to not work with comments. In 
the DOM world, this function should work, but just checking no one 
objects to this change.

Rob