title: mpdf broken images over https
body: <p>I'm using mpdf to generate pdfs. All fine, until I switched to https. After that, pdfs are still correctly generated, but images are broken. Their sources are correctly written with https protocol on the php template. And I also tried to use only relative paths. Nothing.</p>
  
  <p>The following is sample code form my class:</p>
  
  <pre><code>  public function save_pdf($translate = false){
      $this-&gt;mpdf = new \Mpdf\Mpdf();
      $this-&gt;mpdf-&gt;CSSselectMedia='mpdf';
      //$this-&gt;mpdf-&gt;showImageErrors = true; // this will log errors into the php log file
  
      $ch = curl_init($this-&gt;pdf_css_path . '/configurator-pdf.css');
      // this disables ssl check: unsafe
      if($this-&gt;disable_ssl_check) curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
      curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1 );
      curl_setopt($ch, CURLOPT_HEADER, false);
      curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
      $css = curl_exec($ch);
      curl_close($ch);
  
      $template = ($translate) ? $this-&gt;pdf_template_url : $this-&gt;pdf_template_url_it;
      $filename = ($translate) ? $this-&gt;pdf_name : $this-&gt;pdf_it_name;
      $_POST['cid'] = $this-&gt;cid;
  
      $json = json_encode($_POST);
  
      $ch = curl_init($template);
      // this disables ssl check: unsafe
      if($this-&gt;disable_ssl_check) curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
      curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1 );
      curl_setopt($ch, CURLOPT_HTTPHEADER, [ 'Content-Type: application/json', 'Content-Length: ' . strlen($json) ]);
      curl_setopt($ch, CURLOPT_POSTFIELDS, $json);
      $html = curl_exec($ch);
      curl_close($ch);
  
      $this-&gt;mpdf-&gt;WriteHTML($css, 1);
      $this-&gt;mpdf-&gt;WriteHTML($html, 2);
  
      $pdf = $this-&gt;pdf_destination_path . DS . $filename;
      $this-&gt;mpdf-&gt;Output($pdf, \Mpdf\Output\Destination::FILE);
    }
  </code></pre>