title: Python Wand converts from PDF to JPG background is incorrect
body: <p>I found a so wired thing while converting a pdf to jpeg, so i'd like to figure out that maybe this is a small bug.
  See the converted jpg below, you could find that, the background color are all black.
  The image is here: www.shdowin.com/public/02.jpg</p>
  
  <p>However, in the source file of pdf, you can see that the background color are normal white.
  The image is here: www.shdowin.com/public/normal.jpg</p>
  
  <p>I thought this maybe my pdf file's fault, however, when i try to use Acrobat.pdf2image in .NET environment, the converted jpg shows correctly.</p>
  
  <p>Here is my code:</p>
  
  <pre><code>from wand.image import Image
  from wand.color import Color
  import os, os.path, sys
  
  def pdf2jpg(source_file, target_file, dest_width, dest_height):
      RESOLUTION    = 300
      ret = True
      try:
          with Image(filename=source_file, resolution=(RESOLUTION,RESOLUTION)) as img:
              img.background_color = Color('white')
              img_width = img.width
              ratio     = dest_width / img_width
              img.resize(dest_width, int(ratio * img.height))
              img.format = 'jpeg'
              img.save(filename = target_file)
      except Exception as e:
          ret = False
  
      return ret
  
  if __name__ == "__main__":
      source_file = "./02.pdf"
      target_file = "./02.jpg"
  
      ret = pdf2jpg(source_file, target_file, 1895, 1080)
  </code></pre>
  
  <p>Any suggestions for the issue?</p>
  
  <p>I have uploaded the pdf to the url:
  <a href="http://www.shdowin.com/public/02.pdf" rel="noreferrer">02.pdf</a></p>
  
  <p>You can try...</p>