* User1 connects with IPv6
* User2 connects with IPv4 (without any support for IPv6)
* User1 asks via STUN for his address, and receives a IPv6 address back
* User2 asks via STUN for his address, and receives a IPv4 address back
* User2 cannot connect to User1 IPv6 address
After this fix: User1 asks both for his IPv4 and IPv6 addresses.
See #56