clarification needed: https://github.com/adlnet/xAPI-Spec/issues/1065#issuecomment-328794691 tests: https://github.com/RoboSparrow/lx-request/blob/promise/test/xapi/legacy-lrs.js#L260
clarification needed:
adlnet/xAPI-Spec#1065 (comment)
tests: https://github.com/RoboSparrow/lx-request/blob/promise/test/xapi/legacy-lrs.js#L260