title: Validating each property before returning object
body: <p>While using <a href="https://github.com/Mantle/Mantle">Mantle</a>, is there a possibility to before returning the object we are creating (on this case via JSON) to verify that X and Y properties are not nil?</p>
  
  <p>Imagine this class:</p>
  
  <pre><code>@interface Person : MTLModel &lt;MTLJSONSerializing&gt;
  
  @property(nonatomic,strong,readonly)NSString *name;
  @property(nonatomic,strong,readonly)NSString *age;
  
  @end
  </code></pre>
  
  <p>I want a way to verify that if the JSON I received doesn't have the <code>name</code> (for some reason there was an issue on server's DB) I will return a <code>nil</code> Person, as it doesn't make sense to create that object without that property set. </p>