Use `ActiveRecord::Base.primary_key` instead of hardcoding `id` in all caching strategies classes

[stephenchu]

I really think this should be the case. I am unable to use the various strategies as it stands because of this issue.

What do you think?

[orslumen]

Yes, I agree that using the primary_key variable is correct. So far I haven't come across this requirement as the default primary key for Rails models is id.

[stephenchu]

Thanks for the response. The point I was trying to make is that, by using primary_key, you allow greater flexibility in your library for all Rails users.

Anyways, hope to see it in some future commit 🌞