[Ur] Radio button default values

[Adam Chlipala]

Adam Chlipala wrote:
> Taru Karttunen wrote:
>> Is it possible to set a default value for a radio button group
>> (radio/radioOption) without checking any option? I'd like to avoid a
>> nasty error page when a form is submitted without any of the radio
>> buttons checked.
>
> Your question leads me to realize that, as far as I can remember, I've 
> only ever used radio buttons in Ur/Web via one small test program. :)  
> So it's not surprising if the interface has some deficiencies.
>
> What would you think about changing the type of a <radio> widget from 
> [string] to [option string] (so [None] can be the default value)?  
> Does anyone else have an opinion on this?

OK, I've pushed this change.