Bugfix/SMTP connection errors

FIxed SMTP connection errors: invalid HELO hostname (localhost), duplicate HELO (verifier domain).

1. Updated Truemail::Validate::Smtp::Request#run
2. Updated Truemail::Validate::Smtp::Request#session_data
3. Updated behaviour of collecting Truemail::Validate::Smtp::Response instance
4. Updated gem documentation, changelog
5. Updated gem version to 2.0.0

Now helo is a Boolean instead of Net::SMTP::Response instance. It was changed because helo is sending during SMTP-session initializing (Net::SMTP.new.start), and helo is always true if session up is okay. Also hello response won't logged as error if it happens. Example of Truemail::Validate::Smtp::Response instance from 2.x version.

#<struct Truemail::Validate::Smtp::Response:0x00007fa74704cd10
  port_opened=true,
  connection=true,
  helo=true, # Returns Boolean instead of Net::SMTP::Response instance
  mailfrom=false,
  rcptto=nil,
  errors={:mailfrom=>"server response timeout"}>