Bug 1030160 - [Messages][MMS] Subject is considered empty even if it has several empty lines

- Ignore the return key while typing in subject field.
because we use a real event, you can use "assert.ok(event.defaultPrevented)" or check the return value of `dispatchEvent`.

There is no way to check for stopPropagation so you can spy this function on the event object using sinon if we really want to check it (but maybe we don't?)