Extend expireat() to support datetime object?

[charleswhchan]

I was comparing some some code we had written against redis-py that uses the expireat(). The real code works because redis-py supports datetime object as well as int argument.

Currently, mockredis's expireat() function only supports int argument. It would be nice if mockredis could also support a similar interface so that the same code can be tested against a mock backend.
https://github.com/locationlabs/mockredis/blob/master/mockredis/client.py#L208