Add test for union in regression #172

Author: Tarmil

tests/FSharp.SystemTextJson.Tests/Test.Regression.fs

@@ -113,10 +113,14 @@ let ``regression #154`` () =
     |> ignore
 
 type X = { X: X }
+type UY = UY of int * int
+type Y = { Y: UY }
 
 [<Fact>]
 let ``regression #172`` () =
     let x = { X = Unchecked.defaultof<_> }
+    let y = { Y = Unchecked.defaultof<_> }
     let options = JsonSerializerOptions()
     options.Converters.Add(JsonFSharpConverter())
     Assert.Equal("{\"X\":null}", JsonSerializer.Serialize(x, options))
+    Assert.Equal("{\"Y\":null}", JsonSerializer.Serialize(y, options))