Add new HeadersSent API.

src/Microsoft.AspNet.TestHost/ResponseFeature.cs

@@ -30,6 +30,8 @@ namespace Microsoft.AspNet.TestHost
 
         public Stream Body { get; set; }
 
+        public bool HeadersSent { get; set; }
+
         public void OnSendingHeaders(Action<object> callback, object state)
         {
             var prior = _sendingHeaders;
@@ -43,6 +45,7 @@ namespace Microsoft.AspNet.TestHost
         public void FireOnSendingHeaders()
         {
             _sendingHeaders();
+            HeadersSent = true;
         }
     }
 }

test/Microsoft.AspNet.TestHost.Tests/ResponseFeatureTests.cs

@@ -15,6 +15,11 @@ namespace Microsoft.AspNet.TestHost
 
             // Assert
             Assert.Equal(200, responseInformation.StatusCode);
+            Assert.False(responseInformation.HeadersSent);
+
+            responseInformation.FireOnSendingHeaders();
+
+            Assert.True(responseInformation.HeadersSent);
         }
     }
 }