title: Could not cast value of type 'NSNull' to 'NSString' in parsing Json in swift
body: <p>I have the following class </p>
  
  <pre><code>class BannerResponse : NSObject{
  
      let  URL                = "Url";
      let  CONTACT_NO         = "ContactNo";
      let  IMAGE              = "Image";
      let  BIG_IMAGE          = "BigImage";
      let  ID                 = "Id";
      let  TITLE              = "Title";
      let  NO_VIEW            = "NoView";
      let  START_DATE         = "StartDate";
      let  END_DATE           = "EndDate";
  
  
      var url:String;
      var contactNo:String;
      var image:String;
      var bigImage:String;
      var title:String;
      var id:Int;
      var noView:Int;
      var startDate:String;
      var endDate:String;
  
      init(data : NSDictionary){
  
          url         = data[URL] as! String;
          contactNo   = data[CONTACT_NO] as! String;
          image       = data[IMAGE] as! String;
          bigImage    = data[BIG_IMAGE] as! String;
          title       = data[TITLE] as! String;
          id          = data[ID] as! Int;
          noView      = data[NO_VIEW] as! Int;
          startDate   = data[START_DATE] as! String;
          endDate     = data[END_DATE] as! String;
      }
  }
  </code></pre>
  
  <p>when I run the code, I got the following error </p>
  
  <pre><code>Could not cast value of type 'NSNull' (0x10a85f378) to 'NSString' (0x109eccb20).
  </code></pre>
  
  <p><strong>EDIT</strong></p>
  
  <pre><code>do {
  
                              if let json = try NSJSONSerialization.JSONObjectWithData(data!, options: NSJSONReadingOptions.MutableContainers) as? NSDictionary{
  
                                  onSuccess(BannerResponse(data: json))
                              }
                          } catch {
  
                              onFail()
                          }
  </code></pre>