webdav: fixes for RFC/litmus compliance.

- Handles absence of If header.
- Reject Mkcol requests with a body.
- Delete on non-existant item should return 404.
- Support OPTIONS request.
- Ensure logger is invoked for all requests.

See original CL: golang.org/cl/178930043

Change-Id: Ic96aed10c54bb5ed0641092178ad6f15b1440cb4
1 file changed