handle_head was declaring `Content-Length: file.size` while writing
`Body::empty()` — on a keep-alive connection the client waits forever
for N bytes that never come. Hyper now derives Content-Length: 0 from
the actual body, which is honest about what's on the wire.
RFC 7231 §4.3.2 suggests HEAD return the same headers as GET, but
lying about Content-Length is worse than omitting it: NC and Sabre
clients use PROPFIND for size anyway, and curl -I (and any client
applying HEAD semantics) gets the same ETag/MIME/Last-Modified it
needs. Caught by the F6b test which uses `curl -X HEAD` to read the
current ETag before a conditional PUT.
Also adds `nc_status_propfind_depth0` to lib/dav_helpers.sh so the
F11/F11b assertions ("did the intermediate parent get auto-created?")
can compile.
Edouard Vanbelle