channel: Always provide cause code in ChannelHangupRequest.

channel: Always provide cause code in ChannelHangupRequest.

When queueing a channel to be hung up a cause code can be
specified in one of two ways:

1. ast_queue_hangup_with_cause
This function takes in a cause code and queues it as part
of the hangup request, which ultimately results in it being
set on the channel.

2. ast_channel_hangupcause_set + ast_queue_hangup
This combination sets the hangup cause on the channel before
queueing the hangup instead of as part of that process.

In the #2 case the ChannelHangupRequest event would not contain
the cause code. For consistency if a cause code has been set
on the channel it will now be added to the event.

Resolves: #1197