JSDOM doesn't implement the submit method on form
elements. It does have a submit method but this
just fires a 'not implemented' error.
We need to spy on form submissions fired by
clicking on the submit button but can't because
this event calls the submit method internally so
spying on `form.submit` doesn't work.
This adds a helper which spies on the internal
method that is actually called. When JSDOM
implements the submit method properly this should
be removed.
