domish.Element.elements should be called with namespace and element name as 2 arguments,
but it has been confused in several places with the call to `domish.Element.addElement`
which is often done with a `(namespace, name)` tuple. Unfortunately calling with a tuple
is accepted and doesn't raise any error in `elements`, but this result in a wrong element
returned.
This patch fixes the erroneous calls.
8d64157d88