title: PIL convertion to CMYK does not set K component
body: <p>Using pythons PIL from Pillow 5.2 to convert images to CMYK I want to do some rule-of-thumb estimationswith the results.
  The conversion using <code>Image.convert("CMYK")</code> seems to never use the K component. 
  Since I need it for printing purposes. I wish to same save color ink and use black ink whenever it is possible.</p>
  
  <p>I could do these convertion manually using <a href="https://stackoverflow.com/a/30078860/10418139">this code</a> which produces the results I expect. But I do not only have RGB sources and I want to prevent me from converting my sources to RGB by PIL and then use this code to convert it to CMYK.
  Is there a better PIL-native way?</p>
  
  <p>example:</p>
  
  <pre><code>img = Image.open("5procent_gray.png")
  im = img.load()
  print(im[0,0])
  img2 = img.convert("CMYK")
  im2 = img2.load()
  print(im2[0,0])
  </code></pre>
  
  <p>returns</p>
  
  <pre><code>(50, 50, 50)
  (205, 205, 205, 0)
  </code></pre>
  
  <p>instead of <code>(0,0,0,205)</code> which I would expect.</p>