Add Content-Length header to PUT requests

HTTP PUT requires a Content-Length header and many web servers (e.g. nginx)
enforce this limitation. The management utilities swauth-add-{account,user} do
not send Content-Length headers when PUTing objects, which causes requests
routed through nginx to fail.

Fix this by including a Content-Length of zero for all HTTP PUTs.

N.B.: This is not needed for requests performed via make_pre_authed_request().

Signed-off-by: Apollon Oikonomopoulos <apollon@skroutz.gr>