title: is Valid never invoke Custom Validation Attribute
body: <pre><code>public class SomeValidator : ValidationAttribute
  {
      public SomeValidator()
          : base("Message")
      {
  
      }
  
      protected override ValidationResult IsValid(object value, ValidationContext validationContext)
      {
          return new ValidationResult("ERROR");
      }
  </code></pre>
  
  <p>And :</p>
  
  <pre><code>    [SomeValidator]
      public long Something { get; set; }
  </code></pre>
  
  <p>Why isValid method is never invoked ? (I use ASP MVC 3) Thanks for help!</p>