Decode sesssion with_indifferent_access if defined ActiveSupport

[iggant]

Inspire by #71

We are moving from active_record_store to redis_session_store

but we already everywhere use key to access session, so we need this fix

[Jesterovskiy]

#60 @daniel-g @meatballhat I saw this PR and what do you say about @iggant patch? I think this is ok, if someone use ActiveSupport, get with_indifferent_access method. What do you think?

[daniel-g]

it seems fine on my side, I would just add a text in the documentation about this possibility, maybe a "Rails" section.

[iggant]

@Jesterovskiy made rubocop fixes and changes