Channel Properties default value

When there is a default value for a picker field in the ChannlInfo classes, it is impossible to de-select it.

1. Create an archetype project
2. Add Channel info according to http://www.onehippo.org/library/concepts/channels/additional-channel-information.html
3. Upload an asset that matches the default value for the themeCss (/content/assets/themes/css/green.css)
4. Select an asset for the themeCss channel property
5. De-select it

Expected: the picked asset is de-linked

Observed: the default value re-picks the asset