title: Default value optional swift doesnt make it nonOptional
body: <p>I set a default string value with the ?? operator. But i get an optional String?</p>
  
  <pre><code>    request(.GET, CUSTOMER_URL, parameters: params, encoding: .URL).responseJSON { (request, response, result) -&gt; Void in
          var message = JSON(result.value ?? "")["message"].string ?? "Default value to make it nonOptional"
          switch(result) {
          case .Success(let json):
              if let customer = JSON(json)["customer"].dictionaryObject {
                  GlobalCache.sharedInstance.setCustomer(customer)
              }
              completion(succeed: response?.statusCode == 200, message: message)
          case .Failure(_,_):
              completion(succeed: false, message: message)
          }
      }
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/bhA69.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/bhA69.png" alt="message is String?"></a></p>
  
  <p>Expected behaviour</p>
  
  <p><a href="https://i.stack.imgur.com/5B5pX.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/5B5pX.png" alt="enter image description here"></a></p>
  
  <p>Unexpected behaviour</p>
  
  <p><a href="https://i.stack.imgur.com/ybebe.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ybebe.png" alt="enter image description here"></a></p>