Fix invalid cast with DateTimeOffset

This occurred when using DateParseHandling.DateTimeOffset

Src/Newtonsoft.Json/Converters/IsoDateTimeConverter.cs

@@ -136,7 +136,7 @@ namespace Newtonsoft.Json.Converters
             {
 #if !NET20
                 if (t == typeof(DateTimeOffset))
-                    return new DateTimeOffset((DateTime)reader.Value);
+                    return reader.Value is DateTimeOffset ? reader.Value : new DateTimeOffset((DateTime)reader.Value);
 #endif
 
                 return reader.Value;