Restore the ability to use implicit subject in consumer specs

[NotGrm]

Follow-up of #106

To avoid situations when calling subject was having side effects it has been decided that the gem will use an explicit consumer reference to keep track of messages sent to Kafka.

This has the downside to require developers to declare a consumer object (or a named subject) in their tests instead of relying on implicit subjects.

In the issue comment thread, I thought about looking at the described_class and testing if it inherits from Karafka::BaseConsumer. If that is the case the gem's helper could try to look at the consumer referenced object or call the subject reference.

If the described_class is not a Karafka::BaseConsumer the helper will only look for the consumer object

[mensfeld]

@NotGrm can you submit a POC for this as a PR?