Multi-line reply headers not recognized if quoted ones are found below

The fix to #23 seems to have broken reply header handling in emails like [this one](https://github.com/grossvogel/email_reply_parser/blob/multi-line-reply-headers/test/emails/broken_reply_header.txt), containing more reply headers in the quoted text below.

_Expected Behavior_: The first reply header gets its line breaks removed, and ends up in the second fragment along with all of the quoted text.

_Observed Behavior_: The line breaks aren't removed, and therefore it's not recognized as a reply header later on. So the reply header ends up in the first fragment instead of the second.

Start with test/emails/broken_reply_header.txt and review the reply-header handling affected by the fix to #23. Compare the expected and observed fragment behavior in this issue; done means the first reply header has its line breaks removed, is recognized, and appears with the quoted text in the second fragment.

索引モデルが issue の本文から書いたものです。