title: How to string to primitive type in Mantle?
body: <p>I am making application using <a href="https://github.com/Mantle/Mantle" rel="nofollow">Mantle</a> with iOS 9.1 </p>
  
  <pre><code>class SKUser: MTLModel, MTLJSONSerializing {
  
      var userId:NSNumber = -1
      var firstName:String = ""
      var lastName:String = ""
  
      var country:String! = ""
      var address:String! = ""
      var company:String! = ""
      var email:String! = ""
      var mobileNumber:NSNumber = -1
  
      static func JSONKeyPathsByPropertyKey() -&gt; [NSObject : AnyObject]! {
          return [
              "userId": "user_id",
              "firstName": "user_first_name",
              "lastName": "user_last_name",
              "country": "user_country",
              "address": "user_address",
              "company": "user_company",
              "email": "user_email",
              "mobileNumber": "user_mobile_number"
          ]
      }
  }
  </code></pre>
  
  <p>I am not able to convert <strong>mobile number</strong> and <strong>userId</strong> fields to integer or NSNumber, i have tried a lot but could not find solution</p>
  
  <p>Please help me</p>
  
  <p>Thanks in advance...</p>