Merged fix for GRAILS-5528 "useToken in different forms does not evaluate corretly"

Author: graemerocher

src/java/org/codehaus/groovy/grails/web/servlet/mvc/SynchronizerToken.groovy

@@ -42,7 +42,7 @@ class SynchronizerToken implements Serializable {
     }
 
     static SynchronizerToken store(javax.servlet.http.HttpSession session) {
-        SynchronizerToken token = new SynchronizerToken()
+        SynchronizerToken token = session.getAttribute(KEY) ?: new SynchronizerToken()
         session.setAttribute(KEY, token)
         return token
     }