kio_http: avoid double Content-Type and Depth when used by KDAV

Summary: This makes libkdav 19.08 work again after commit 9713ea02e49eda.

Test Plan: syncing a CALDAV calendar in korganizer no longer fails

Reviewers: vkrause

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24880