Skip events that already have default prevented

[dgreif]

There may be scenarios where you want to do client-side validation or have some other specialty handler for a form. In these scenarios, it would be nice to have an escape hatch that prevents remoteForm from handling the event.

There are also some rare (but possible) edge cases with code bundlers where an app could double load this module and execute it twice, creating two registered listeners for form submit events. In this scenario, both instances of remoteForm would trigger their handlers and send the fetch request, causing a double submission.

In both cases, a solution is to check if the event has already been handled by checking event.defaultPrevented. This allows custom handlers to prevent further action with the event, and handles the double load scenario because we already call preventDefault within remoteForm.

This should fix an outstanding issue in github/github where comment creation and other actions can be executed twice.

Note: We may need to consider this a breaking change as it could prevent forms from submitting in apps that are already preventing default somewhere. Not sure how we typically handle unintended side-effects like this 😅

[koddsson]

LGTM