Issue #1206 - DefaultBodyModelValidator throws if get accessor throws.
Fix: The MvcOptions takes in a list of ExcludeFromValidationDelegate (Func<Type,bool>). This func verifies if the type is excluded in validation or not.
Fixing a few comments relating to DefaultBodyModelValidator.
[Fixes #1332] Port IContentNegotiator to WebAPI shim for CoreCLR support

1. Ported IContentNegotiator.
2. Enabled Core CLR configuration