From cf10e69f17a202499aed8de6dd08f9642ab61b53 Mon Sep 17 00:00:00 2001
From: Ian Lance Taylor HTTPS_PROXY is unset.
+ The Transport
+ type has a new field
+ GetProxyConnectHeader
+ which may be set to a function that returns headers to send to a
+ proxy during a CONNECT request.
+ In effect GetProxyConnectHeader is a dynamic
+ version of the existing field
+ ProxyConnectHeader;
+ if GetProxyConnectHeader is not nil,
+ then ProxyConnectHeader is ignored.
+