[System]: Disallow "https" proxies in ServicePointManager.FindServicePoint().

(cherry picked from commit e734cd8b598a0c2d036db66c049f7b7c1f5fda2c)

mcs/class/System/System.Net/ServicePointManager.cs

@@ -328,7 +328,7 @@ namespace System.Net
 				usesProxy = true;
 				bool isSecure = address.Scheme == "https";
 				address = proxy.GetProxy (address);
-				if (address.Scheme != "http" && !isSecure)
+				if (address.Scheme != "http")
 					throw new NotSupportedException ("Proxy scheme not supported.");
 
 				if (isSecure && address.Scheme == "http")