Property serialization for Link fails on document serialization

[kalfa]

With the (amended) test code in README, the current code fails with:

Traceback (most recent call last):
File "./tests.py", line 188, in testComplexLinks
self.doc = self.xrd.to_xml().documentElement
File "/home/kalfa/src/recipy/python-xrd/xrd.py", line 484, in to_xml
return _render_xml(self)
File "/home/kalfa/src/recipy/python-xrd/xrd.py", line 313, in _render_xml
node.setAttribute('type', prop.type)
AttributeError: 'tuple' object has no attribute 'type'

A fix is being proposed, with regression tests.