Fix omniauth-oauth2 1.4 callback_url regression

Bug introduced by omniauth-oauth2 change: omniauth/omniauth-oauth2#70
Fixes #3

Author: jtokoph

lib/omniauth/strategies/twitch.rb

@@ -52,7 +52,11 @@ def build_access_token
       def access_token_options
         options.access_token_options.inject({}) { |h,(k,v)| h[k.to_sym] = v; h }
       end
-
+  
+      def callback_url
+        full_host + script_name + callback_path
+      end
+  
       def authorize_params
         super.tap do |params|
           options[:authorize_options].each do |k|