Updating task failed (dates)

### What Happened?

Trying to save a task with start and due date gives an error.
The task has a start and a due date that I configured in Nextcloud tasks app.
The due date is after the start date, otherwise it's not possible to create the tasks.

Test task has a start date of 11th october 2021 17:00 (5pm) and a due date of 15th november 2021 13:59 (1:59pm).
I printed some debug output in the `TaskModel.update_task` method to get the date information. I use the methods `task.get_dtstart ().get_value ().as_ical_string ()` and `task.get_icalcomponent().get_due ().as_ical_string ()`. Both give me the value of `20211115T135901`, which corresponds to the due date.

### Steps to Reproduce

1. Create task with start and due date in Nextcloud Tasks app
2. Sync with Tasks
3. Open the task and click save without changing anything
4. A error window opens

### Expected Behavior

Working normaly

(Happens with compiled version from git and with the version from appcenter)

### OS Version

6.x (Odin)

### Software Version

Compiled from git

### Log Output

```shell
HTTP-errorcode 415 (Unsupported Media Type): Sabre\DAV\Exception\UnsupportedMediaType
Validation error in iCalendar: DUE must occur after DTSTART[exception][message]
```

### Hardware Info

Latitude-E7270
Intel® Core™ i7-6600U CPU @ 2.60GHz
Intel® HD Graphics 520 (Skylake GT2)