Add singleSignOutFilter bean configuration

This is already in the master branch

SpringSecurityCasGrailsPlugin.groovy

@@ -121,6 +121,18 @@ class SpringSecurityCasGrailsPlugin {
 			println '\nConfiguring Spring Security CAS ...'
 		}
 
+		if (conf.cas.useSingleSignout) {
+
+			// session fixation prevention breaks single signout because
+			// the service ticket is mapped to the session id which changes
+			conf.useSessionFixationPrevention = false
+
+			singleSignOutFilter(SingleSignOutFilter) {
+				ignoreInitConfiguration = true
+				casServerUrlPrefix = conf.cas.serverUrlPrefix
+			}
+		}
+
 		SpringSecurityUtils.registerProvider 'casAuthenticationProvider'
 		SpringSecurityUtils.registerFilter 'casAuthenticationFilter', SecurityFilterPosition.CAS_FILTER