Add test for #115

Not sure why #115 has no test, but it appears not to.

``` python
walker = html5lib.getTreeWalker("lxml")
tree = html5lib.parseFragment("a", treebuilder="lxml")
assert list(walker(tree)) == [{'data': 'a', u'type': 'Characters'}]
```

That should work, but we probably want to test with all tree walkers.
