compatibility fixes for gvfs-webdav and Javascript clients

Smaller changes which improve compatibility with gvfs-webdav and JS clients

- HttpResponseMultiStatus needs an XML content type
- base_url and path incorrectly set when requesting http://server/djangodav_mountpoint (without trailing slash)
- dont return displayname=None for root resource
- no error 500 when client sends "Content-Length:" header (instead of "Content-Length:0")