Allow user to nullify selected value and remove restriction to force select first option for select inputs

Hey,

Please review this PR and let me know if this make sence for you or advice for better solution. If it's OK, I will go ahead and add tests

I use it in survey workflow on the last page. We ask customer to rate a produc. And we want to allow him to change his choise and allow to put 0 in rating from 0 to 5. I don't want to show 0 option as a star, it's weird. This is how it looks now http://i.imm.io/12o54.png When he clicks on selected rating I reset it to zero but don't show zero value(as a star) - second commit here.