Add Content-Length 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} and
also middleware.Swauth.handle_put_account do not specify Content-Length with
PUT requests, 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().