Always use with_indifferent_access

[Vasfed]

In a typical rails app the probability of ActiveSupport being already present is almost certain.
Do we need additional checks?

Formally this is a major change, since someone may have hacks in place to avoid with_indifferent_access (like requiring us before rails) and may require some workaround like

class DifferentAccessRedisSessionStore < RedisSessionStore
  def session_default_values
    [generate_sid, {}]
  end

  def decode(data)
    serializer.load(data)
  end
end

Rails.application.config.session_store(DifferentAccessRedisSessionStore, redis: ...)

[hogelog]

I am not the maintainer but I think this Pull Request is appropriate.

The current redis-session-store uses ActionDispatch::Session::AbstractSecureStore, but this class depends on ActiveSupport. So there is probably no one using the current redis-session-store without ActiveSupport::HashWithIndifferentAccess.
https://github.com/rails/rails/blob/v5.2.4.1/actionpack/lib/action_dispatch/middleware/session/abstract_store.rb#L6
https://github.com/rails/rails/blob/v5.2.4.1/actionpack/lib/action_dispatch/middleware/cookies.rb#L3-L6

I think we should merge this pull request to remove this branch for code simplicity. @Jesterovskiy ?