Fix wrong set initialisation. If a string is given it treats each character as an element. We want to treat the string as a one value hence we have to wrap in a list.

Summary: This would overestimate active compared to highly engaged since most of the dates have at least four different characters. This does not explain though other drops. I am leaning more and more on the side that the data we have is correct and the number has dropped throughout the summer? I cannot find other error with the script

Test Plan: run student_teacher_count.q

Reviewers: jace

Reviewed By: jace

Differential Revision: http://phabricator.khanacademy.org/D4191