Make null option selected

When the item is null, the null option needs to indicate it’s selected.
Without this, the browser assumes that the value of the select list has
changed since the page loaded. This causes issues with the “Save Before
Exit” plugin.
Merge pull request #27 from rimblas/master

Make null option selected.
I believe this might break the placeholder functionality in certain cases. I'll need to test this first before releasing.