More often than not I find myself passing a single Validation error in the Result.Invalid method. This PR addresses that.
I do believe this to be a needed improvement, as it's very annoying to create new Lists only to pass a single object in them.

• Support for single validation error in the Result.Invalid method
• Added unit tests to validate behavior
• All existing behavior remains the same. The single instance of the Validation Error will be passed in a new List upon Result.Invalid creation

     //This instantiates a new list with the single validation error in it
     public new static Result Invalid(ValidationError validationError)
        {
            return new Result(ResultStatus.Invalid) { ValidationErrors = { validationError } };
        }