title: Multi-part POST with file and string in HTTPClient 4.1
body: <p>I need to create Multi-part POST request containing fields:
  <code>update[image_title] = String
  update[image] = image-data itself</code>.
  As you can see both are in associative array called "update".
  How could I do it with HTTPClient 4.1, because I found only examples for 3.x line of this library.  </p>
  
  <p>Thank you in advance.</p>