email: multiline encoded headers

Bug report

There seems to be a problem with multiline encoded headers, for example subject headers.

from email import message_from_string, policy
import base64

e = """From: <test@test.com>
To: <test@test.com>
Subject: =?UTF-8?B?0JfQsNGB0YLRgNCw0YXQvtCy0LrQsCAi0JDQu9C40LDQvdGGINCX0LTRgNCw0LLQ?=
 =?UTF-8?B?tSIg4oCTINCY0L3QtNC40LLQuNC00YPQsNC70LXQvSDQvdC+0LzQtdGAINC90LAg?=
 =?UTF-8?B?0YPQstC10LTQvtC80LvQtdC90LjQtQ==?=

test"""

e = message_from_string(e, policy=policy.default)

s1 = e["Subject"]

try:
    e.as_bytes()
except Exception:
    print("error")
else:
    print("ok")

s = (base64.b64decode("0JfQsNGB0YLRgNCw0YXQvtCy0LrQsCAi0JDQu9C40LDQvdGGINCX0LTRgNCw0LLQ") + base64.b64decode("tSIg4oCTINCY0L3QtNC40LLQuNC00YPQsNC70LXQvSDQvdC+0LzQtdGAINC90LAg")
    + base64.b64decode("0YPQstC10LTQvtC80LvQtdC90LjQtQ==")).decode("utf-8")

del e["Subject"]
e["Subject"] = s

s2 = e["Subject"]

try:
    e.as_bytes()
except Exception:
    print("error")
else:
    print("ok")

print(s1 == s2)

e = """From: <test@test.com>
To: <test@test.com>
Subject: =?UTF-8?B?0JfQsNGB0YLRgNCw0YXQvtCy0LrQsCAi0JDQu9C40LDQvdGGINCX0LTRgNCw0LLQtSIg4oCTINCY0L3QtNC40LLQuNC00YPQsNC70LXQvSDQvdC+0LzQtdGAINC90LAg0YPQstC10LTQvtC80LvQtdC90LjQtQ==?=

test"""

e = message_from_string(e, policy=policy.default)

s3 = e["Subject"]

try:
    e.as_bytes()
except Exception:
    print("error")
else:
    print("ok")

print(s2 == s3)

Your environment

Tested on Python 3.10.4 and 3.11.0a7 on Arch Linux

使用 Python 的 email.message_from_string、policy.default 和 as_bytes 入口运行提供的复现代码。比较 s1、s2 和 s3，包括序列化失败，然后跟踪 email 包对折叠的编码 Subject 标头的处理。完成标准是：多行和单行编码标头的行为一致且没有错误，并且解码后的值相匹配。