Fix a bug introduced by new omniauth-oauth2 (1.4.0)

This basically reverts the commit recently introduced on the Oauth2
omniauth gem here:

omniauth/omniauth-oauth2#70
omniauth/omniauth-oauth2@2615267

The callback_url in the base omniauth gem includes the query string so
by removing the overridden version the query string was being included
which was making Doorkeeper complain about a callback URL not being
equal.

Author: tpett

lib/omniauth/strategies/watermark.rb

@@ -23,6 +23,10 @@ class OmniAuth::Strategies::Watermark < OmniAuth::Strategies::OAuth2
     }
   end
 
+  def callback_url
+    full_host + script_name + callback_path
+  end
+
   def raw_info
     @raw_info ||= access_token.get('/api/v1/me.json').parsed
   end