Don't use localeToFixed for solution

Test Plan:
Try the following using comma or decimal point. Also try the English version.
http://localhost:8000/exercises/multiplying_decimals_1.html?seed=70&problem=0&debug&lang=es-ES

Reviewers: eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D8054