Do not set `url-gateway-method' in `url-https'. (Bug#16543)

Currently, when `url-retrieve' is called for an https url it modifies
the variable `url-gateway-method'.  This has been changed to
explicitly pass the requested gateway method to other functions.

When `url-retrieve' is being processed then (via
`accept-process-output') another `url-retrieve' call from a pending
timer can be started.  The second call would always see the modified
`url-gateway-method' of the first one, which in general does not match
the url.

2014-09-28  Ulf Jasper  <ulf.jasper@web.de>

	* url-gw.el (url-open-stream): New optional parameter
	`gateway-method'.  If non-nil use it instead of global variable
	`url-gateway-method'.

	* url/url-http.el (url-http): New optional parameter
	`gateway-method', pass it to `url-http-find-free-connection'.
	(url-http-find-free-connection): New optional parameter
	gateway-method, pass it to `url-open-stream'.
	(url-https-create-secure-wrapper): Do not modify
	`url-gateway-method' but explicitly provide 'tls as gateway-method
	parameter to `url-https'.