Fix mapping proficiencies for display. We need to create new object since we want to reuse the raw data

Summary: Fix NaN error related to changing object without copying it

Test Plan: at exercise-summary after changing date the proficiency table should not have NaNs as it was before

Reviewers: chris

Reviewed By: chris

CC: jace

Differential Revision: http://phabricator.khanacademy.org/D2454