GH-243) Allow explicit proxy override

Allow passing a specific proxy into the proxy cache that is used no
matter what.

src/Core/Http/ProxyCache.cs

@@ -18,6 +18,7 @@ public class ProxyCache : IProxyCache
         /// settings.
         /// </summary>
         private static readonly IWebProxy _originalSystemProxy = WebRequest.GetSystemWebProxy();
+        private IWebProxy _overrideProxy;
 
         private readonly ConcurrentDictionary<Uri, WebProxy> _cache = new ConcurrentDictionary<Uri, WebProxy>();
 
@@ -55,6 +56,11 @@ public static ProxyCache Instance
 
         public IWebProxy GetProxy(Uri uri)
         {
+            if (_overrideProxy != null)
+            {
+                return _overrideProxy;
+            }
+
 #if !BOOTSTRAPPER
             // Check if the user has configured proxy details in settings or in the environment.
             WebProxy configuredProxy = GetUserConfiguredProxy();
@@ -134,6 +140,15 @@ 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;
+            }
         }
 
         private static WebProxy GetSystemProxy(Uri uri)