Bug 1030417 - aria role key for number keyboard keys

Keys in number keyboard now have role="key".
The problem was: special keys shouldn't have role="key", so it was checking that key element has 'special-key' CSS class in order to add the role attribute or not. But seems that 'special-key' CSS class is used also for other kind of keys (for example number keys in the number layout)
I'm checking against the isSpecialKey function instead, that seems more reliable to know if the key is really a special key or not.