amqp_channel: Adapt channel_exit handling

... after `rabbit_writer` conversion to gen_server.

[Why]
The `rabbit_writer` being a regular gen_server now, it will stop with
the reason `{shutdown, Reason}`. Otherwise this would be considered a
crash by the supervision tree and it would log even more scary errors
and stacktraces.

[How]
If the reason is `{shutdown, _}`, we just unpack the embedded reason and
use it for the channel stop reason. This ensures the behavior of the
channel process doesn't change.

Author: dumbbell

deps/amqp_client/src/amqp_channel.erl

@@ -839,7 +839,11 @@ handle_channel_exit(Reason = #amqp_error{name = ErrorName, explanation = Expl},
     handle_shutdown({connection_closing, ReportedReason}, State);
 handle_channel_exit(Reason, State) ->
     %% Unexpected death of a channel infrastructure process
-    {stop, {infrastructure_died, Reason}, State}.
+    Reason1 = case Reason of
+                  {shutdown, R} -> R;
+                  _             -> Reason
+              end,
+    {stop, {infrastructure_died, Reason1}, State}.
 
 handle_shutdown({_, 200, _}, State) ->
     {stop, normal, State};