Fix HEAD tempurls

HEAD-only tempurls didn't work; tempurl only allowed a HEAD request if
the tempurl was generated for GET or PUT (that is, the method in the
HMAC-signed string was "GET" or "PUT").

The intent of the code was to allow a user with a GET or a PUT tempurl
to also perform a HEAD request; I think the breaking of HEAD tempurls
is just a bug.

Change-Id: I621ddaac03e0d058dd9e7c7c374cb5c4b6386d36
Merge "Fix HEAD tempurls"