v4.23.0p2

• Variable improvement (remove redundant substitution)
• Remove Sisimai::RFC2606 (Unused module)
• MIME decoding improvement #130 Thanks to @outreach-soren.
  • Implement Sisimai::MIME.makeflat
  • Implement Sisimai::MIME.breaksup
  • Call Sisimai::MIME.makeflat at Sisimai::Message::Email.parse
  • Other related updates in Sisimai::Bite::Email::*
• Tiny improvement in Sisimai::String.to_plain