Add code to fill in default values for new radio buttons added to forms ...

[mgoertzen]

...which already have entries.

[jegelstaff]

This is an interesting approach, and the method on the data handler should be moved to the element class I think, so it could be used more generally/cleanly. It's an element maintenance issue, not a raw data access issue.

[jegelstaff]

See also: https://github.com/jegelstaff/formulize/pull/203