lib/bugsnag/middleware/rack_request: early load session for Rails 4

Fixes #144 (https://github.com/bugsnag/bugsnag-ruby/issues/144)

Context: https://github.com/rails/rails/issues/10813
@kyrylo Why do we need to check if the session is loaded given that we already load it? What happens if you just call `session.to_hash`?