Update XML boolean format.

http://www.w3.org/TR/xmlschema-2/#boolean
> An instance of a datatype that is defined as ·boolean· can have the following legal literals {true, false, 1, 0}.

```php
array('true' => true, 'false' => false)
```

**current format**
```xml
<xml><true>1</true><false/></xml>
```
`false` is formatted to empty string.

**this format**
```xml
<xml><true>true</true><false>false</false></xml>
```
or
```xml
<xml><true>1</true><false>0</false></xml>
```

Signed-off-by: Haruaki Otake <aaharu@hotmail.com>