Fix for connecting harmonizely.com to Baikal

[larseggert]

See https://help.harmonizely.com/en/articles/4602263-connecting-baikal-calendar-works-but-the-availability-is-not-loading and https://github.com/sabre-io/dav/issues/1244#issuecomment-582607462 for details.

(I did not come up with this workaround - @Yannik did - but I can confirm that it made things work for me.)

[wetneb]

This fix also works for me. Maintainers, what do you think of it? @phil-davis

[ByteHamster]

I'm a bit worried that this might cause issues with other clients

[wetneb]

The link to the new help page is now https://help.simplymeet.me/index.php/Connecting_Baikal_calendar_works_but_the_availability_is_not_loading