[xml-bindings]Minor bug in python bindings tree stuff

[Pete Ryland <peter_ryland reech com>]

Hi there,

I hope this is the right forum to post this too.

The docSetRootElement() function in the python bindings is wrong (IMO) 
on two counts.  Firstly, it should be a method of doc rather than node, 
and secondly it will raise an exception when the C function 
xmlDocSetRootElement() returns null which will happen under non-error 
conditions since it "Returns : the old root element if any was found" 
and this could well be null.

This makes it quite hard to create a document from scratch in python.  A 
workaround goes something like this:

doc = libxml2.newDoc('1.0')
rootNode = doc.newDocNode(None, 'myrootnode', None)
# rootNode.docSetRootElement(doc)
libxml2mod.xmlDocSetRootElement(doc._o, rootNode._o)
...

All the best,
Pete (not on the list, i.e. please CC me on replies)