Issue title: Fuzzy dates are not kept after restarting GTG
 Issue body: If I create a new task with a fuzzy due date ("soon") or set an existing task to such a due date, it is displayed correctly.
Closing and re-opening GTG deletes this fuzzy date and sets an empty due date.

Debug script gives no errors.
Absolute due dates are permanent.
 Comments: 
 Comment 0: Yeah, can confirm. I was able to reproduce this bug on the master branch.
But to be sure, on which version of GTG did this happen to you?

 Comment 1: It's the actual master branch.
I don't know how to get the actual version, but right now "git show" says

commit e793b87790f461e182b9fddf8870275d98e0ae11 (HEAD -> master, origin/master, origin/HEAD)
{'source': 'GithubIssues', 'repo_name_with_owner': 'getting-things-gnome/gtg', 'issue_id': 825424136, 'repo_stars': 545.0}