Compatibility with omniauth-oauth2 >= 1.4

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

lib/omniauth/strategies/greenhouse.rb

@@ -20,6 +20,18 @@ class Greenhouse < OmniAuth::Strategies::OAuth2
       def raw_info
         @raw_info ||= access_token.get('/v1/partner/current_user').parsed
       end
+
+      # Override callback URL for compatibility with omniauth-oauth2 >= 1.4,
+      #   which by default passes the entire URL of the callback, including
+      #   query parameters. Greenhouse fails validation because that doesn't match the
+      #   registered callback as required in the Oauth 2.0 spec.
+      # Refs:
+      # https://tools.ietf.org/html/rfc6749#section-4.1.3
+      # https://github.com/omniauth/omniauth-oauth2/commit/26152673224aca5c3e918bcc83075dbb0659717f
+      # https://github.com/omniauth/omniauth-oauth2/pull/70
+      def callback_url
+        options[:redirect_uri] || (full_host + script_name + callback_path)
+      end
     end
   end
 end