Handle comma separated roles

Certain LTI providers will provide multiple roles, comma separated. For example, here's what was returned via Sakai.

`roles=Instructor,Administrator,urn:lti:instrole:ims/lis/Administrator,urn:lti:sysrole:ims/lis/Administrator`

This method will still give us a single element array if there is no commas and fix the issue with no roles being properly detected.