Fix a broken unit test

For who is intersted by the resolution:
 - a channel cannot be considered opened until the endpoint answered
 - the code was sending 2 events in a row...
 - the first event will open the channel
 - but if the second event arrive "too fast", the channel wont be "fully
   opened"

The solution:
 - wrap the channel in a BufferedChannel, which automatically prevent
   sending more than one message until the channel is opened fully.