Rework add_data

[CaptainN]

The method for setting the data of the OnceForm does two different things, and that is confusing. Figure out a better way.

1. If you don't set the second param, OnceForm will polyfill the data array and set all fields from the data array (including ones marked as non-enumerable or private).
2. If you set the second param false, OnceForm will only set public (enumerable) fields.

The second behavior is probably what you expect when you invoke this method directly. OnceForm will automatically invoke the method the first way, when it detects a post back has occurred. The first behavior is currently the default.

Suggestion: separate the two behaviors in two methods to make it clearer.