#668 use extension method to create object instance

This is a fix for #668, where if you use a DTO without a parameterless constructor it silently errors and returns an empty response. 

I've made the functionality in XmlMetaDataHandler the same as that in JsonMetadataHandler. Instead of trying to create an object instance using Activator.CreateInstance, which is what causes the original error, we create one use the CreateInstance extension method. Which already handles parameterless constructors.