Update ke substate, make inexact actually work

Summary:
Make perseus send undefined instead of false to answer-types so that
inexact works. The alternative is trying to differentiate "" and `false` in
answer-types, which isn't very easy.

Test Plan:
Make a perseus problem with a number answer type. Set the answer to
"1" and make sure inexact is unchecked. Check that "1" is accepted, and "1.05"
is not. Now, check inexact, and set max error to "0.1". Check that "1" and
"1.05" are accepted.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2825