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.

Written by the indexing model from the issue text.