Update canvas_user_activities.rb

when there is no source for activity, the source should default to bCourses instead of Canvas
Update canvas_user_activities_spec.rb

CLC-2473 - when there is no emitter, source should revert to bCourses instead of Canvas
Merge pull request #1615 from jonmhays/master

CLC-2473 - Update canvas_user_activities.rb