BYE is not sent when 200/OK is not ACK-ed

[riza]

Scenario:

1. Make call from device A to device B
2. Answer the call on device B and disconnect immediately.
3. In case device B disconnected the call (hangup) and ACK is lost, BYE will not be send and call will not be disconnected.

In case device B didn't hangup, timeout will occur and BYE is sent
(200/OK is not ACK-ed).

However, on the above case, B hangup and got delayed until ACK is received.
When the timeout occur, the stack tried to hangup but still got delayed.
This is because, BYE can only be sent on a confirmed dialog (see #1712) also the hangup cause cannot be changed due to this check on inv_set_cause

(cause_code > inv->cause)

Thanks to Nir Lavi for the report.

[riza]

In 5104:

Re #1857: (Fix) BYE is not sent when 200/OK is not ACK-ed