Consider the usage of WebDAV methods sufficient for assuming WebDAV

Summary:
So far WebDAV only really works when using webdav[s]: URLs, which is an
unexpected KIO-specific detail that does not seem to be understood by all
applications (see e.g. the hacks in KDAV::DavManager). With this change
at least the WebDAV methods also emit the WebDAV specific headers when
using http[s] URLs.

Reviewers: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24316