rails3: fix normalization for values comparison in quiz submissions api

in rails3: 0.0.to_json => "0.0"
in rails2: 0.0.to_json => "0"
Since we're comparing the values and json strings this was breaking the test

Change-Id: I65d517cd9f0707e75f66b436d9dabb324fb09293
Reviewed-on: https://gerrit.instructure.com/30859
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>
QA-Review: Derek DeVries <ddevries@instructure.com>