script_load cannot work with bytes

[PCManticore]

Currently if you try to pass a bytes object to script_load you'll get a similar error to the following:

  ...
  File "/Users/claudiu/.pyenv/versions/3.6.5/lib/python3.6/site-packages/mockredis/client.py", line 1322, in script_load
    sha_digest = sha1(script.encode("utf-8")).hexdigest()
AttributeError: 'bytes' object has no attribute 'encode'

It seems that script_load is currently expecting to be called with a string object here: https://github.com/locationlabs/mockredis/blob/master/mockredis/client.py#L1333

But script_load in redis can work with both bytes and strings, which should probably be allowed in mockredis as well.