Patch for ActionMailer: Allow body to be specified for nested parts

http://dev.rubyonrails.org/ticket/10271

I submitted this patch the other day. Basically, right now the code
will throw an exception if you try to specify a body and a nested
part:

part :content_type => "multipart/alternative", :content_disposition =>
"inline", :body => "Nothing to see here." do |p|
      p.part :content_type => "text/html", :body => "<b>test</b>
HTML<br/>"
end

This patch includes what is specified in the body as a separate part,
as if it had been nested.

I need some +1's - or feedback about why not.