Support IUnvalidated + IEnumerable ValueProviders

This will fix #696 

When the original ValueProvider implements the `IEnumerableValueProvider` in combination with the `IUnvalidatedValueProvider` then only the `IEnumerableValueProvider` was proxied, which means that it is not possible anymore for the MVC framework to check if it is possible to get the unvalidated value in case a viewmodel property would be decorated with the `AllowHtml` attribute, resulting in the exception mentioned in #696 

Since the `AlternateType` solution is based on proxying only one type, I created an `IUnvalidatedEnumerableValueProvider` and corresponding `UnvalidatedEnumerableValueProvider` that will implement the original `IUnvalidatedValueProvider` and the original `IEnumerableValueProvider` and which will forward those calls to the original value provider that implemented both interfaces, which fixes the issue