title: ios NSMutableAttributedString NSKernAttributeName wrong kerning with "fl" and "fi" letters
body: <p>I'm using <code>attributed strings</code> on labels to change the spacing between letters:</p>
  
  <pre><code>var attributedString=NSMutableAttributedString(string: "Fifi floflo")
  attributedString.addAttribute(NSKernAttributeName, value: CGFloat(10), range: NSRange(location: 0, length: attributedString.length))
  label.attributedText=attributedString
  </code></pre>
  
  <p>The kerning is working well excepts for two groups of letters which are "<strong>fl</strong>" and "<strong>fi</strong>" (maybe others I didn't see at the moment).
  I'm using "Avenir-medium" font but I tested with basic Arial font and the problem still occurs.</p>
  
  <p><a href="https://i.stack.imgur.com/bNDyH.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/bNDyH.png" alt="With the word &#39;Profile&#39;"></a></p>
  
  <p><a href="https://i.stack.imgur.com/IcExk.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/IcExk.png" alt="With the word &quot;Fifi floflo&#39;"></a></p>