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.

从 test/emails/broken_reply_header.txt 开始，检查受 #23 修复影响的回复标头处理。比较此 issue 中 fragment 的预期行为和实际观察到的行为；完成的标准是：第一个回复标头的换行符被移除、能够被识别，并与引用文本一起出现在第二个 fragment 中。

由索引模型根据 Issue 内容生成。