bug#53412: wrappin text lines is not enough, headers shoould be split too

[Francesco Potortì]

FP:
>>>>> If there is an overlong header, other headers which are utf-8
>>>>> escaped are not sent correctly, and the recipient sees something
>>>>> like this:
>>>>>
>>>>> Subject: test =?utf-8?Q?=C3=A0?= b c
>>>>>  =?utf-8?Q?=C3=AC?=

LI:
>>>>That looks correct to me?

FP:
>>> No, if that's what the recipent sees in their inbox.  I don't know
>>> why, but mail sent by Emacs with an overlong header (whichever header)
>>> ends up with recipients seeing that string in their MUA.

LI:
>> They see literally that displayed?  Then it sounds like the mail you're
>> sending out has been double-rfc2047-encoded.

AS:
>I would think this happens because of the call to mml-to-mime in the
>mail-send-hook he uses.

Sure, that's it.  This happens when there is an overlong header line and 
mml-to-mime is in the mail-send-hook.

mml-to-mime in the mail-send-hook is there because without it overlong text 
lines would go unbroken