Not detecting quoted content in emails coming from Mail.app

See https://github.com/jdpopkin/email_reply_parser/commit/bf9d5ce00f52f9096acf87a60f3ed6134338f03e for an example of this.

Start by reviewing the Mail.app example in the linked commit and tracing how this library handles quoted email content. Confirm the current parser behavior against that example, then add coverage and make the example’s quoted section detectable without changing unrelated parsing behavior.

索引モデルが issue の本文から書いたものです。