Fix issue #106, "enter" key doesn't trigger form submit in modals by default.

In most browsers, when you hit the enter key while a form element is
focused the browser will trigger the form 'submit' event.  Google
Chrome also does this, but not when when the default submit button is
hidden with 'display: none'.

This patch provides a workaround the issue.
Add a test for submitting a form on a modal with the enter key.

This is a basic test for the code introduced in
commit 8477d976890aba5b2659586f663f4aae3472bdf8.