Fix scrubbing issue

Solves errors like :
ERROR:planet.runner:TypeError: 'NoneType' object has no attribute '__getitem__'
ERROR:planet.runner:  File "/home/olivier/svn/svn.debian.org/python-apps/planet-venus/git-svn/planet-venus/planet/spider.py", line 480, in spiderPlanet
    writeCache(uri, feed_info, data)
ERROR:planet.runner:  File "/home/olivier/svn/svn.debian.org/python-apps/planet-venus/git-svn/planet-venus/planet/spider.py", line 167, in writeCache
    scrub.scrub(feed_uri, data)
ERROR:planet.runner:  File "/home/olivier/svn/svn.debian.org/python-apps/planet-venus/git-svn/planet-venus/planet/scrub.py", line 149, in scrub
    node['value'] = ''.join([str(token) for token in tree])
ERROR:planet.runner:  File "/home/olivier/svn/svn.debian.org/python-apps/planet-venus/git-svn/planet-venus/planet/vendor/html5lib/serializer/htmlserializer.py", line 199, in serialize
    for token in treewalker:
ERROR:planet.runner:  File "/home/olivier/svn/svn.debian.org/python-apps/planet-venus/git-svn/planet-venus/planet/vendor/html5lib/filters/optionaltags.py", line 18, in __iter__
    type = token["type"]