Fix compliance with original recv_into()

[hongquan]

Per Python 3 documentation, the second parameter of recv_into is optional.

[mindflayer]

Good job, I noticed that a couple of days ago when I was investigating the problem with redis.
Thank you.

[coveralls]

Coverage decreased (-0.6%) to 97.642% when pulling 0176f16 on sunshine-tech:fix/recv_into into 6dbfdec on mindflayer:master.

[mindflayer]

Here it is:
https://github.com/mindflayer/python-mocket/releases/tag/3.7.1
https://pypi.org/project/mocket/3.7.1/

@hongquan

[hongquan]

Thanks, @mindflayer