add spec for instantaneous and unsolicited PUBACKS

the case is already handled by my original refactor for allowing
multiple outstanding packets, but this adds the spec to be sure.

see njh#158

Author: ccutrer

spec/mqtt/client_spec.rb

@@ -741,6 +741,20 @@ def register_for_ack(packet)
       expect(client).to receive(:send_packet) { |packet| expect(packet.id).to eq(2) }
       client.publish "topic", "message", qos: 1
     end
+
+    it "does not crash when receiving a PUBACK for a packet it never sent" do
+      expect { client.send(:handle_packet, MQTT::Packet::Puback.new(id: 666)) }.not_to raise_error
+    end
+
+    it "does not crash with QoS 1 when the broker sends the PUBACK instantly" do
+      allow(client).to receive(:send_packet).and_wrap_original do |send_packet, packet, *args, **kwargs, &block|
+        send_packet.call(packet, *args, **kwargs, &block).tap do
+          client.send(:handle_packet, MQTT::Packet::Puback.new(id: packet.id))
+        end
+      end
+
+      expect { client.publish("topic", "message", qos: 1) }.not_to raise_error
+    end
   end
 
   describe "#subscribe" do