Fix LTI max_score method.

This PR adds fix for LTI's `max_score` method.

When LTI module is not graded (attribute `graded=False`) method `max_score` returns value of `weight` attribute, that is `1.0` by default.
It should not be so, because it affects on total score (each additional module adds +1 to the total max score). If LTI module is not gradable `0` should be returned.