Bug 870603 - [MMS] Multi-recipient: completing # or auto-search enter with ; or ENTER erases content

Looks like the event constructor was renamed from KeyboardEvent to KeyEvent at some point. Instead of relying on an instanceof check, just test the event.type for /^key/

https://bugzilla.mozilla.org/show_bug.cgi?id=870603
Signed-off-by: Rick Waldron <waldron.rick@gmail.com>