Update shared/libraries/Field_behavior.php

修复选择 编辑器(TEXT) 字段类型出错问题:
Error Number: 1101
BLOB/TEXT column 'content' can't have a default value
ALTER TABLE `dili_u_m_content` ADD `content` TEXT DEFAULT '' NOT NULL
Filename: D:\wamp\web\suiyu.cc\system\database\DB_driver.php
Line Number: 330