* lib/net/smtp.rb (Net::SMTP#data): enable buffering while
  'data' send for optimizing Net::SMTP#send_message.
  [ruby-dev:48329] [misc #9981]
  patch by Masahiro Tomita.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46793 b2dd03c8-39d4-4d8f-98ff-823fe69b080e