Backport new Session.getKeys() from v5 to v4

Author: goldfeder

src/main/java/com/googlecode/objectify/impl/Session.java

@@ -4,6 +4,7 @@
 import com.googlecode.objectify.util.ResultNow;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.Set;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 
@@ -69,4 +70,11 @@ public void clear() {
 	public String toString() {
 		return map.toString();
 	}
+
+	
+	/**
+	 * @return all the keys currently in the session. If you really want this data, subclass ObjectifyImpl and
+	 * use the protected getSession() method.
+	 */
+	public Set<Key<?>> keys() { return map.keySet(); }
 }