Use message context as inbound context when available

This way middlewares are able to mutate the message context and have
that get threaded through to our invoker

v2/client/invoker.go

@@ -127,6 +127,9 @@ func (r *receiveInvoker) IsResponder() bool {
 
 func computeInboundContext(message binding.Message, fallback context.Context, inboundContextDecorators []func(context.Context, binding.Message) context.Context) context.Context {
 	result := fallback
+	if mctx, ok := message.(binding.MessageContext); ok {
+		result = mctx.Context()
+	}
 	for _, f := range inboundContextDecorators {
 		result = f(result, message)
 	}