Fix bug: Wrong Backgroung to PDF Thumbnails

lib/thumbnail/creation.rb

@@ -43,6 +43,7 @@ module Thumbnail
       # computes the new image geometry and yields to a block. The
       # return value of the block is the return value of the method.
       img = Magick::Image.read(input)[0]
+      img.alpha(Magick::DeactivateAlphaChannel)
       img.change_geometry!(size) { |cols, rows| img.thumbnail! cols, rows }
       img.write(output)
     end