Fix ASP.NET HttpResponse not to depend on a (fixed) bug in UriBuilder

* System.Web/System.Web/HttpResponse.cs: Ensure '?' is not encoded as
%3F while redirecting (e.g. aspxerrorpath) since this can cause a loop.
i.e. not found leading to not found...

* System/System/Test/System/UriBuilderTest.cs: Add test to confirm the
right behavior of UriBuilder wrt how it was used by Mono's ASP.NET

mcs/class/System.Web/System.Web/HttpResponse.cs

@@ -880,10 +880,16 @@ namespace System.Web
 				HttpRuntimeSection config = WebConfigurationManager.GetWebApplicationSection ("system.web/httpRuntime") as HttpRuntimeSection;
 				if (config != null && config.UseFullyQualifiedRedirectUrl) {
 					var ub = new UriBuilder (context.Request.Url);
-					ub.Path = url;
+					int qpos = url.IndexOf ('?');
+					if (qpos == -1) {
+						ub.Path = url;
+						ub.Query = null;
+					} else {
+						ub.Path = url.Substring (0, qpos);
+						ub.Query = url.Substring (qpos + 1);
+					}
 					ub.Fragment = null;
 					ub.Password = null;
-					ub.Query = null;
 					ub.UserName = null;
 					url = ub.Uri.ToString ();
 				}

mcs/class/System/Test/System/UriBuilderTest.cs

@@ -343,6 +343,36 @@ namespace MonoTests.System
 			Assert.AreEqual (80, ub.Port, "2.Port");
 			Assert.AreEqual ("/dir/subdir/file", ub.Path, "2.Path");
 		}
+
+		[Test]
+		public void AspNetRedirectUsage_Old ()
+		{
+			Uri uri = new Uri ("http://192.168.0.21:80/WebResource.axd?d=AAAAAAAAAAEAAAAAAAAAAA2");
+			UriBuilder ub = new UriBuilder (uri);
+			ub.Path = "error404.aspx?aspxerrorpath=/WebResource.axd";
+			ub.Fragment = null;
+			ub.Password = null;
+			ub.Query = null;
+			ub.UserName = null;
+			// a bug in older UriBuilder did not encode the ? - existing ASP.NET depends on buggy behavior
+			Assert.AreEqual ("http://192.168.0.21/error404.aspx%3Faspxerrorpath=/WebResource.axd", ub.Uri.ToString ());
+		}
+
+		[Test]
+		public void AspNetRedirectUsage_New ()
+		{
+			string path = "error404.aspx?aspxerrorpath=/WebResource.axd";
+			Uri uri = new Uri ("http://192.168.0.21:80/WebResource.axd?d=AAAAAAAAAAEAAAAAAAAAAA2");
+			UriBuilder ub = new UriBuilder (uri);
+			int qpos = path.IndexOf ('?');
+			ub.Path = path.Substring (0, qpos);
+			ub.Fragment = null;
+			ub.Password = null;
+			ub.Query = path.Substring (qpos + 1);
+			ub.UserName = null;
+			// this is what ASP.NET really means (the ?)
+			Assert.AreEqual ("http://192.168.0.21/error404.aspx?aspxerrorpath=/WebResource.axd", ub.Uri.ToString ());
+		}
 	}
 }