fix display of FIMB quiz questions on rails 3

fixes CNVS-13790

string manipulation just lost the html_safe flag on rails 3, but not
rails 2

test plan:
 * create a quiz with a fill in multiple blanks question
 * preview it
 * it should render the question correctly, not have a bunch
   of raw html

Change-Id: I399874707a880e55c3772e5a780ff595de9062d6
Reviewed-on: https://gerrit.instructure.com/37188
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Ahmad Amireh <ahmad@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>