Revert "minor refactoring: use public visible API instead on calling …

…internal private stuff"

This reverts commit 1eb54d2, that causes SchemalessSpec to fail.

grails-datastore-core/src/main/groovy/org/grails/datastore/mapping/core/AbstractSession.java

@@ -253,7 +253,7 @@ public boolean isDirty(Object instance) {
             return false;
         }
 
-        Object entry = getCachedEntry(persister.getPersistentEntity(), id, true);
+        Object entry = getEntryCache(instance.getClass(), false).get(id);
         return ((NativeEntryEntityPersister)persister).isDirty(instance, entry);
     }