Close opened image files

[hidakatsuya]

This PR fixes #29. See description of #29 for details.

require 'prawn/emoji'

Prawn::Document.generate 'sushi.pdf' do
  font 'DejaVuSans.ttf'
  text '🐟 / 🔪 + 🍚 / 🍾 = 🍣'
end

ObjectSpace.each_object(File) do |f|
  puts "%s: %d" % [f.path, f.fileno] unless f.closed?
end

DejaVuSans.ttf: 26
prawn-emoji-3.1.0/emoji/images/1f35a.png: 29
prawn-emoji-3.1.0/emoji/images/1f37e.png: 30
prawn-emoji-3.1.0/emoji/images/1f363.png: 31
prawn-emoji-3.1.0/emoji/images/1f41f.png: 27
prawn-emoji-3.1.0/emoji/images/1f52a.png: 28

prawn-emoji just passes the path of image file to Prawn#image method but not open image file.

prawn-emoji/lib/prawn/emoji/drawer.rb

Line 53 in 26f83a1

@document.image image.path, at: [x, y], width: image.width

Image files are opened by prawn. See prawnpdf/prawn#975 for details. This issue has been already fixed by prawnpdf/prawn#1090, but not released yet.

The GC should close the file, but fix it so that it explicitly closes the file.