Permit setting version on mock fetch response

[eapache]

@sanchezl this should address #938.

[sanchezl]

Thanks @eapache. Would you consider an additional API such as:

func (mfr *MockFetchResponse) SetMaxVersion(kafkaVersion KafkaVersion) *MockFetchResponse {
switch {
  case kafkaVersion.IsAtLeast(sarama.V0_11_0_0): // e.g. when supported
    mfr.version = 3
  case kafkaVersion.IsAtLeast(sarama.V0_10_0_0):
    mfr.version = 2
  default:
    mfr.version = 1
}
return mfr
}

[eapache]

Hmm, actually would it just be simpler to return the same version of FetchResponse as was received in the FetchRequest? That would be simpler, but I'm not sure if they line up in all cases?

[sanchezl]

What about just adding property to the MockBroker itself (e.g. MockBroker.SetMessageFormat(kafkaVersion KafkaVersion) that would correspond to setting the log.message.format.version property on a real kafka server.

I advocate for the use of KafkaVersion, not just the message version since that is what the log.message.format.version property expects.

[eapache]

I advocate for the use of KafkaVersion, not just the message version since that is what the log.message.format.version property expects.

AFAICT log.message.format.version refers to the version of the actual Message data struct, not the Request/Response wire protocol. They are versioned differently.

[eapache]

I've reviewed this some more and I think the PR I have here is correct. The wire protocol version should be flexible even knowing the broker version.