Allow redis to set expires with an int arg

[davegaeddert]

It seems like there are at least a couple scenarios in controller.py where the expires can be an int instead of a datetime.

https://github.com/ionrock/cachecontrol/blob/3bd694551d28445723c18f4ad8c44bce55ff9b5f/cachecontrol/controller.py#L325
https://github.com/ionrock/cachecontrol/blob/3bd694551d28445723c18f4ad8c44bce55ff9b5f/cachecontrol/controller.py#L349

With the way that works currently, I think some check needs to be done in RedisCache to see what type we're working with... the other solution that comes to mind is that expires should only be an int or a datetime? That would be a more involved change but I'd be willing to put some time into it @ionrock if you prefer it goes that direction. Implementing mypy or something might help sort out that kind of bug.

Fixes #268

[davegaeddert]

#251 (documentation around expires value) is also related to this.

[ionrock]

@davegaeddert I think you're right about normalizing how we treat dates, but I also don't have a good idea of a direction to take. The ints seem better to work with overtime, but I could be convinced otherwise. If you're interested in trying to normalize things, I'd be grateful, but this change looks like a good start. Thanks!

[davegaeddert]

Sounds good, thanks for merging it!

I'm not sure I know enough to push on normalizing... from some other issues it sounds like people also use custom cache backends and I'm not sure how to best navigate that.