fake_redis should raise a ResponseError when given a non-int value for "expire"

[wimkeir]

There appears to be a behaviour mismatch between "real" Redis and fake_redis. When given a command like EXPIRE key 1.5, Redis raises a ResponseError, claiming that the value is not an int. fake_redis should also raise an error when expire() is passed a seconds parameter that is not an int. We should also include a test to assert that this error is raised.