fix: handle return type correctly

Author: dbrattli

test/TestBuiltins.fs

@@ -13,4 +13,4 @@ let ``test __name__ works`` () = __name__ |> equal "test_builtins"
 [<Fact>]
 let ``test write works`` () =
     let result = builtins.``open``("test.txt", OpenTextMode.Write)
-    result.write "ABC" |> equal ()
+    result.write "ABC" |> equal 3