add: support EHLO command and reply with enabled extensions

lib/midi-smtp-server.rb

@@ -293,8 +293,16 @@ def process_line(line)
  Thread.current[:ctx][:server][:helo] = @cmd_data
  # set sequence state as RSET
  Thread.current[:cmd_sequence] = :CMD_RSET
- # reply ok
- return "250 OK"
+ # check whether to answer as HELO or EHLO
+ if line =~ /^EHLO/i
+ # reply supported extensions
+ return "250-8BITMIME\r\n" + 
+ (@auth_mode != :AUTH_FORBIDDEN ? "250-AUTH LOGIN PLAIN\r\n" : "") +
+ "250 OK"
+ else
+ # reply ok only
+ return "250 OK"
+ end
 
  when (/^AUTH(\s+)((LOGIN|PLAIN)(\s+[A-Z0-9=]+)?|CRAM-MD5)\s*$/i)
  # AUTH