Refactor the Consumer unit tests
Refactor the Consumer unit tests

* Call the internal consumer "kafka_consumer" instead of "mock_consumer.
* Prefix all lets that contain Java objects "kafka_"
* Use "partitions" not "topic_partitions" in all tests.
* Default to an RSpec double since most of the tests can't use MockConsumer.
* Move the MockConsumer to a shared context and include it where a MockConsumer is used.
burtcorp/kafka-clients-jruby,burtcorp/kafka-clients-jruby