title: Model validation attribute [Required] doesn't accept a string of all spaces
body: <p>I'm working with ASP.Net MVC 4 website project.</p>
  
  <p>When I set the attribute <code>Required</code> for a model property.</p>
  
  <pre><code>[Display(Name = "Some Model Property:")]
  [Required]
  public string SomeModelProperty{ get; set; }
  </code></pre>
  
  <p>This will mark the input field to be red when its value is empty.</p>
  
  <p>My issue is that this field is also marked red when its value is <strong>all spaces</strong></p>
  
  <p>I want to allow input value to have all spaces only for a <code>Required</code> property.</p>
  
  <p>How can I get to that?</p>