NullReferenceException when not CodedException

When the exception is not a CodedException, I was trying to generate an
error response based on a null reference.
Create ValidateModel attribute

In order to do basic view model validation in Web API, I created an
attribute that looks at the model state and returns an error response
(401) if there are any validation errors.
Make JSON the default media type for text/html

If the client does not specify how they want to view the Web API, JSON
will be returned by default.
Pass HttpRequestMessage as Parameter

Instead of using the ApiController's Request property, I am passing the
HttpRequestMessage as a parameter to the action. I changed to unit tests
to correspond.