SimpleConsumer.consume raises 'NoneType' object has no attribute 'value' while deserializer is used.

[kamilglod]

In case of nullable ret consume method should not run self._deserializer method on None value.

https://github.com/Parsely/pykafka/blob/2.8.0/pykafka/simpleconsumer.py#L510

PyKafka version: 2.8.0