Check if request is sent from official ownCloud client

There are authentication backends such as Shibboleth that do send no Basic Auth credentials for DAV requests. This means that the ownCloud DAV backend would consider these requests coming from an untrusted source and require higher levels of security checks. (e.g. a CSRF check)

While an elegant solution would rely on authenticating via token (so that one can properly ensure that the request came indeed from a trusted client) this is a okay'ish workaround for this problem until we have something more reliable in the authentication code.