Keys command doesn't work with django-redis 4.9.0

[jbagot]

Since django-redis is in the version 4.9.0 the pattern to find with the command keys is an object called CacheKey. MockRedis doesn't expect this object it's expecting an string and then crash.
With this line it's fixed: pattern = self._encode(pattern)
I'll do a pull requests with it

[jbagot]

I created a Pull Request #139 to solve this problem