[reSIProcate] crossing byes in dum

[Jason Fischl <jason@xxxxxxxxxxxxxx>]

I'm just  writing some test cases where two dum user agents hang up 
simultaneously. The current behavior is:

A INVITEs B
A and B both send BYE at same time
A rejects BYE with 481
B rejects BYE with 481

Is this valid behavior or should one of the user agents send a 200? It 
seems to me that it is probably ok as it is.

Jason