jQuery版本升级, 大于1.6.1之后，
$("#element").attr("checked"， true) 能够设置#element的attr有checked,
但是设置$("#element").attr("checked"， false),
再调用$("#element").attr("checked") 的返回值为undifined
根据官方文档：
jQuery 1.6.1 Released
| Attribute/Property | .attr() | .prop() |
|---|---|---|
| accesskey | ✓ | |
| align | ✓ | |
| async | ✓ | ✓ |
| autofocus | ✓ | ✓ |
| checked | ✓ | ✓ |
| class | ✓ | |
| contenteditable | ✓ | |
| defaultValue | ✓ | |
| draggable | ✓ | |
| href | ✓ | |
| id | ✓ | |
| label | ✓ | |
| location * | ✓ | ✓ |
| multiple | ✓ | ✓ |
| nodeName | ✓ | |
| nodeType | ✓ | |
| readOnly | ✓ | ✓ |
| rel | ✓ | |
| selected | ✓ | ✓ |
| selectedindex | ✓ | |
| src | ✓ | |
| style | ✓ | |
| tabindex | ✓ | |
| tagName | ✓ | |
| title | ✓ | |
| type | ✓ | |
| width ** | ✓ |
可以发现checked应该使用prop, 即：
$("#element").prop("checked", false)
$("#element").prop("checked")
