Restore from Rack breaking API change

Now all methods of Rack::Auth::AbstractRequest returns strings, so, let's use to_sym for comparison
https://github.com/rack/rack/commit/0c76175fcccad74ba2f991c487d3669c28a297c8

Please check other comparisons in your code
Here you use to_s ... WHY!? :scream:
https://github.com/nov/rack-oauth2/blob/master/lib/rack/oauth2/server/resource/bearer.rb