fix bug #11335.

mcs/class/System.ServiceModel.Web/System.ServiceModel.Web/OutgoingWebRequestContext.cs

@@ -36,6 +36,7 @@ namespace System.ServiceModel.Web
 	{
 		internal OutgoingWebRequestContext ()
 		{
+			Headers = new WebHeaderCollection ();
 		}
 
 		public string Accept { get; set; }

mcs/class/System.ServiceModel.Web/Test/System.ServiceModel.Web/WebOperationContextTest.cs

@@ -61,6 +61,9 @@ namespace MonoTests.System.ServiceModel.Web
 				Assert.IsNotNull (WebOperationContext.Current.IncomingRequest, "#4");
 				Assert.IsNotNull (WebOperationContext.Current.IncomingResponse, "#5");
 				Assert.IsNotNull (WebOperationContext.Current.OutgoingResponse, "#6"); // pointless though.
+
+				Assert.IsNotNull (WebOperationContext.Current.OutgoingRequest.Headers, "#7");
+				Assert.AreEqual (0, WebOperationContext.Current.OutgoingRequest.Headers.Count, "#8");
 #endif
 			}
 			ch.Close ();