net/protocol.rb: fix SMTP dot stuffing

* net/protocol.rb (using_each_crlf_line): fix SMTP dot-stuffing
  for messages not ending with a new-line.
  [ruby-core:61441] [Bug #9627] [fix GH-616]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46060 b2dd03c8-39d4-4d8f-98ff-823fe69b080e