Add rack 2 check

lib/omniauth/builder.rb

@@ -2,7 +2,7 @@ module OmniAuth
   class Builder < ::Rack::Builder
     def initialize(app, &block)
       @options = nil
-      if rack14?
+      if rack14? || rack2?
         super
       else
         @app = app
@@ -15,6 +15,10 @@ module OmniAuth
       Rack.release.split('.')[1].to_i >= 4
     end
 
+    def rack2?
+      Rack.release.start_with? '2.'
+    end
+
     def on_failure(&block)
       OmniAuth.config.on_failure = block
     end