title: Allow empty string for EmailAddressAttribute
body: <p>I have property in my <code>PersonDTO</code> class:</p>
  
  <pre><code>[EmailAddress]
  public string Email { get; set; }
  </code></pre>
  
  <p>It works fine, except I want to allow empty strings as values for my model, if I send JSON from client side:</p>
  
  <pre><code>{ Email: "" }
  </code></pre>
  
  <p>I got <code>400 bad request</code> response and </p>
  
  <pre><code>{"$id":"1","Message":"The Email field is not a valid e-mail address."}
  </code></pre>
  
  <p>However, it allows omitting <code>email</code> value:</p>
  
  <pre><code>{ FirstName: "First", LastName: 'Last' }
  </code></pre>
  
  <p>I also tried:</p>
  
  <pre><code>[DataType(DataType.EmailAddress, ErrorMessage = "Email address is not valid")]
  </code></pre>
  
  <p>but it does not work.</p>
  
  <p>As far as I understood, <code>Data Annotations Extensions</code> pack does not allow empty string either.</p>
  
  <p>Thus, I wonder if there is a way to customize the standard <code>EmailAddressAttribute</code> to allow empty strings so I do not have to write custom validation attribute.</p>