Allow overwriting Mail::Message sort order… for real - #853

So I tried to build the following email:

mail = Mail.new do
  # FIXME: This does not work.
  body.set_sort_order []

  part do |p|
    p.text_part { body 'Hello world!' }
    p.html_part { body 'Hello <b>world</b>!' }
  end

  attachments['README.txt'] = 'some text'
end

An alternative part (plain + HTML) followed by a .txt attachment.

The resulting email was structured this way:

multipart/mixed
  text/plain (attachment)
  multipart/alternative
    text/plain
    text/html

Starting with the attachment looked like a bug to me, so I dug a bit. I found about Mail::Body#sort_parts! and the default @part_sort_order, so I tried disabling the sorting altogether by passing Mail::Body#set_sort_order an empty array (see FIXME comment above).

It did not work, because Mail::Message#ready_to_send! does a hard-coded parts.sort! with the default order, effectively making Mail::Body#set_sort_order useless.

This patch contains the fix. The example above now produces the following structure:

multipart/mixed
  multipart/alternative
    text/plain
    text/html
  text/plain (attachment)