Fix so that we can use toJson: () => this.props

Summary:
Atm, we'd have to use _(this.props).omit("onChange", "ref") for
things to work properly. This is at least a first step in changing that.

I found this 'bug' when I used _(this.props).omit("onChange") for numberline
and grading didn't work because it had ref: "widget" from the editor and
grading calls this.refs[id] where the id isn't "widget" (it was "number-line 1").

Test Plan: Nothing should have changed. Test that nothing changed.

Reviewers: jack

Reviewed By: jack

CC: alpert, alex

Differential Revision: http://phabricator.khanacademy.org/D7249