Imported Upstream version 6.0.0.172

Former-commit-id: f3cc9b82f3e5bd8f0fd3ebc098f789556b44e9cd

external/corefx/src/System.Net.Requests/src/System/Net/HttpWebRequest.cs

@@ -1139,11 +1139,22 @@ namespace System.Net
 
                 Debug.Assert(handler.UseProxy); // Default of handler.UseProxy is true.
                 Debug.Assert(handler.Proxy == null); // Default of handler.Proxy is null.
+
+                // HttpClientHandler default is to use a proxy which is the system proxy.
+                // This is indicated by the properties 'UseProxy == true' and 'Proxy == null'.
+                //
+                // However, HttpWebRequest doesn't have a separate 'UseProxy' property. Instead,
+                // the default of the 'Proxy' property is a non-null IWebProxy object which is the
+                // system default proxy object. If the 'Proxy' property were actually null, then
+                // that means don't use any proxy. 
+                //
+                // So, we need to map the desired HttpWebRequest proxy settings to equivalent
+                // HttpClientHandler settings.
                 if (_proxy == null)
                 {
                     handler.UseProxy = false;
                 }
-                else
+                else if (!object.ReferenceEquals(_proxy, WebRequest.GetSystemWebProxy()))
                 {
                     handler.Proxy = _proxy;
                 }