Instead of including the etag in the HTTP GET request and letting the
server set the Response Code depending on whether the etag machines, this
makes the client first issue a HEAD request, which is uses to get the etag
and the file size. We need to do the HEAD beforehand anyway to get the
file size for resumable downloads, and this approach prevents the server
from using the etag as a form of tracking cookie:
http://lucb1e.com/rp/cookielesscookies/closes#562
2550329ab5