Do not alllow serializing Task's

Author: mythz

src/ServiceStack.Text/Common/JsWriter.cs

@@ -340,9 +340,15 @@ public void WriteValue(TextWriter writer, object value)
             var valueWriter = (IValueWriter)value;
             valueWriter.WriteTo(Serializer, writer);
         }
+        
+        void ThrowTaskNotSupported(TextWriter writer, object value) =>
+            throw new NotSupportedException("Serializing Task's is not supported. Did you forget to await it?");
 
         private WriteObjectDelegate GetCoreWriteFn<T>()
         {
+            if (typeof(T).IsInstanceOf(typeof(System.Threading.Tasks.Task)))
+                return ThrowTaskNotSupported;
+            
             if (typeof(T).IsValueType && !JsConfig.TreatAsRefType(typeof(T)) || JsConfig<T>.HasSerializeFn)
             {
                 return JsConfig<T>.HasSerializeFn

tests/ServiceStack.Text.Tests/SpecialTypesTests.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections;
 using System.IO;
+using System.Threading.Tasks;
 using NUnit.Framework;
 using ServiceStack.Web;
 
@@ -122,5 +123,13 @@ public void Does_not_serialize_Streams()
             var dto = new RawRequest { Id = 1, RequestStream = new MemoryStream() };
             Serialize(dto, includeXml: false);
         }
+
+        [Test]
+        public void Serializing_Tasks_throws_NotSupportedException()
+        {
+            Assert.Throws<NotSupportedException>(() => Task.FromResult(1).ToJson());
+            Assert.Throws<NotSupportedException>(() => Task.FromResult(1).ToJsv());
+        }
+
     }
 }