Mail::Body#sort_parts! sideeffect

it "should detect text_part and remain consistent" do
  m = Mail.new(:content_type => 'mixed/multipart')
  p = Mail::Part.new(:content_type => 'mixed/alternative')
  p.add_part(Mail::Part.new(:content_type => 'text/html', :body => 'HTML TEXT'))
  p.add_part(Mail::Part.new(:content_type => 'text/plain', :body => 'PLAIN TEXT'))
  a = Mail::Part.new(:content_type => 'text/plain', :body => 'signature')
  m.add_part(p)
  m.add_part(a)
  m.text_part.body.decoded.should == 'PLAIN TEXT'
  m.to_s
  m.text_part.body.decoded.should == 'PLAIN TEXT' #fail is "signature"
end