core: fix calls to `domish.Element.elements`: 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.

-r requirements.txt

pytest
pytest_twisted