Add the config.session_key to override 'rack.session'

[nicoolas25]

Hello,

This PR allows this kind of configuration:

OmniAuth.config.session_key = "rack.production.session"

Maybe some of you will find it usefull as I do ^^

[yui-knk]

This is not instance variable in origin code, I feel maybe there are some reason to not use instance variable (but author only knows...).

[nicoolas25]

It isn't nope. It seems to use the default attribute accessor so it basically does the same thing. I updated it for clarity and it does not break the test suite, that's good enough for me ^^

[nicoolas25]

Since Rack isn't yet allowing you to update the env's key where the session is stored (rack.session) this have a very small use case.

I'll close it but I think the cleaning in #818 may be worth merging.

[pas256]

I don't understand how to use this?
How do I inject something into the Rack env so that I can then use this?
For background, what I am doing is creating the OAuth redirects in my Rails 5 API only backend, so there is no session.