Due date changes not synced

[stacksjb]

I've discovered that if an assignment due date is updated by a professor in Canvas, the tool will not sync the updated due date, since it already sees that the task has been synced.

This isn't frequent but is a problem when it occurs.

[scottquach]

I've made an update that will update the todoist item if the canvas due date is different than the todoist due date

[andreicozma1]

@scottquach

if (task['due'] and task['due'].get('date') != assignment.get('due_date'))

Might need to be:

if (task['due'] and task['due'].get('date') != assignment.get('due_at')):

'due_date' is an invalid key therefore the result comes out as None

[andreicozma1]

Raised a PR with fix on #13