Don't break backward compatibility.  It's not like this was reported as a bug that caused a problem for real world code, it is about theoretical consistency.  The risk of breaking someone code is much higher than the benefit of any such consistency, when talking about a bug fix.

Aside from that, however, I see request.('GET', '/') and request.('GET', '/', '') as clearly *different* from an API call standpoint, so I would in any case preserve the existing behavior.