Just thought I'd let the maintainers for this gem know of this issue I'm facing when using the omniauth-steam gem together with Rails 4.1 with the new cookies_serializer option set to JSON.
The OpenID object used by the ruby-openid gem dependency/library cannot be serialized as JSON and this causes all sorts of exceptions to occur downstream. The solution is to switch the setting in config/initializers/cookies_serializer.rb to :marshal.
More details here: openid/ruby-openid#75 (comment)
I'm not sure if there's a way to avoid using ruby-openid and I'm not sure if the omniauth-steam gem is right place to address or even discuss this issue, but I thought I'd leave this here if anyone else comes looking for a solution.