fix memory cache Put method should set value when key exist.

@@ -53,11 +53,7 @@ func (bc *MemoryCache) Put(name string, value interface{}, expired int64) error
 		Lastaccess: time.Now(),
 		expired:    expired,
 	}
-	if _, ok := bc.items[name]; ok {
-		return errors.New("the key is exist")
-	} else {
-		bc.items[name] = &t
-	}
+	bc.items[name] = &t
 	return nil
 }