title: sizeToFit() not working properly with attributedText
body: <p>Here is my cellForRowAtIndexPath<br></p>
  
  <pre><code>func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -&gt; UITableViewCell {
  
          //cell init
          //set values to cell property
          let str2 = NSString(format: "%@", (displayArray[indexPath.row].valueForKey("username") as? String)!)
          let attrStr2 = NSMutableAttributedString(string: str2 as String)
          attrStr2.addAttribute(NSFontAttributeName, value:UIFont(name: "HelveticaNeue-Bold", size: 12)!, range: NSMakeRange(0, str2.length ))
          topCell.username.attributedText = attrStr2
  
          topCell.username.sizeToFit()
  
          topCell.title.backgroundColor = UIColor.redColor()
          topCell.username.backgroundColor = UIColor.yellowColor()
          //some calculation which is not affecting username.frame.size.width
          return topCell;
  
  }
  </code></pre>
  
  <p>and here is my tableView<br>
  <a href="https://i.stack.imgur.com/8OXdR.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/8OXdR.png" alt="enter image description here"></a>
  <br>You can see label with yellow background, 
  And yes, autolayout is disable, so I think my code is fine but not working as expected.</p>
  
  <p>Please help me to solve this issue.
  Thanks in advance </p>