Allow HEAD for POST tempurls

HEAD requests are already allowed for GET and PUT tempurls; this
commit adds that for POST tempurls. Since POST replaces all the
object's metadata, it's quite useful to be able to HEAD the object in
order to fetch the old metadata and do a client-side merge of the new
metadata (like with normal, token-authenticated POST requests).

Change-Id: I603c7822cd27f0e304fd27024f83f95114eb0aef
Merge "Allow HEAD for POST tempurls"