(GH-8) Accept null as proxy override

This commit fixes null rejection as argument for
ProxyCache.Override method. It should be allowed because this
value represents empty proxy (according to MSDN Documentation).

src/Core/Http/ProxyCache.cs

@@ -19,6 +19,7 @@ public class ProxyCache : IProxyCache
         /// </summary>
         private static readonly IWebProxy _originalSystemProxy = WebRequest.GetSystemWebProxy();
         private IWebProxy _overrideProxy;
+        private bool _isProxyOverridden = false;
 
         private readonly ConcurrentDictionary<Uri, WebProxy> _cache = new ConcurrentDictionary<Uri, WebProxy>();
 
@@ -56,13 +57,14 @@ public static ProxyCache Instance
 
         public IWebProxy GetProxy(Uri uri, bool bypassProxy)
         {
+
             if (bypassProxy) return null;
 
-            if (_overrideProxy != null)
+            if (_isProxyOverridden)
             {
                 return _overrideProxy;
             }
-            
+
 #if !BOOTSTRAPPER
             // Check if the user has configured proxy details in settings or in the environment.
             WebProxy configuredProxy = GetUserConfiguredProxy();
@@ -142,15 +144,20 @@ public void Add(IWebProxy proxy)
             {
                 _cache.TryAdd(webProxy.Address, webProxy);
             }
-        }        
-        
+        }
+
         public void Override(IWebProxy proxy)
         {
-            var webProxy = proxy as WebProxy;
-            if (webProxy != null)
-            {
-                _overrideProxy = webProxy;
-            }
+            _overrideProxy = proxy;
+            _isProxyOverridden = true;
+
+        }
+
+        public void ClearOverriding()
+        {
+            _overrideProxy = null;
+            _isProxyOverridden = false;
+
         }
 
         private static WebProxy GetSystemProxy(Uri uri)