Fix unicode error in subsection

@chrisndodge this fixes a bug brought up on the mailing list, see my response to this: https://groups.google.com/forum/#!topic/edx-code/cdsQafpz0aA

@wedaly I don't know the best place to add a test for creating a course with sections, subsections, and problem names with Unicode, but obviously this is a needed test! Can you please advise?