done.txt uses the wrong syntax for completed tasks, if task had no creation date

[Bellamonte]

If a task had no creation date (e.g. if you switched off the corresponding setting of this addon) and you complete it, it is written to done.txt with only the completion date.
The syntax rules for completed task though destine to have also a creation date, if you have a completion date (which itself is obligatory for completed tasks).
See https://github.com/todotxt/todo.txt/blob/master/README.md