title: valueForKey throws error for optional value, Swift
body: <p>I have simple class <code>User</code>. When I try to call <code>valueForKey</code> where key is optional value I get an error. See below:</p>
  
  <pre><code>class User : NSObject{
      var name = "Greg"
      var isActive:Bool?        
  }
  
  var user = User()
  user.isActive = true // initiated
  
  var ageValue : AnyObject! = user.valueForKey("name")// OK
  
  var isActive : AnyObject! = user.valueForKey("isActive") //ERROR
  </code></pre>
  
  <blockquote>
    <p>&lt;__lldb_expr_862.User 0x7fa321d0d590> valueForUndefinedKey:]: this class is not key value coding-compliant for the key isActive.</p>
  </blockquote>
  
  <p>If I'll initiate the value, it will work:</p>
  
  <pre><code>var isActive:Bool =  true
  </code></pre>
  
  <p>How to make it work with optional values?</p>