IE7 compatibility for prototype.js version of Chosen single select

When using prototype.js version of Chosen in single select mode, evt.target is not always an extended object in container_mousedown. To achieve IE7 compatibility, the attached patch is required. Otherwise the expand arrow of the single select can not be used for that browser. See attached screenshot.

![Ska rmavbild 2012-12-19 kl 10 32 07](https://f.cloud.github.com/assets/2294247/21614/9de17734-49c0-11e2-8245-6f59d1224270.png)