fix #292 duplicate data with jsonp

> [JSFiddle](http://jsfiddle.net/but0xmfy/)

> ![Screenshot](http://i.imgur.com/dmPYKyZ.png)

> Looks like https://github.com/lhorie/mithril.js/blob/481d3feeada70667deb53ef98eeb4458b8caba16/mithril.js#L846 should be removed.

[Updated JSFiddle](http://jsfiddle.net/but0xmfy/1/)

![Updated screenshot](http://i.imgur.com/tu1MPye.png)