Default value of "request_timeout_ms" is inconsistent between doc and code

https://github.com/dpkp/kafka-python/blob/master/kafka/consumer/group.py#L86 
```
request_timeout_ms (int): Client request timeout in milliseconds.  Default: 40000.
```

https://github.com/dpkp/kafka-python/blob/master/kafka/consumer/group.py#L253
```
'request_timeout_ms': 305000, # chosen to be higher than the default of max_poll_interval_ms
```

