Description:
------------
In function 'mail()', a 'Warning' occurred depending on the combination of mail-headers to be specified, and not be sent.
But, if at least one is missing, it succeed.

That actual elements, please see the test script.

I know that "mb_send_mail()" does not support "quoted-printable".
However, I think that there is a discrepancy in behavior.


Emvironment
    Apache/2.4.27 (Win64) PHP/5.6.36, and, Apache/2.4.27 (Win64) PHP/7.0.30 
    Target MTA is IIS SMTP Service.



Test script:
---------------
<?php
error_reporting(E_ALL|E_STRICT);
ini_set("display_errors", "On");
echo mb_language('uni');

$sendto = "address@example.com";
$subject = "[Test]Subject";
$msg = "=E3=83=86=E3=82=B9=E3=83=88=E6=9C=AC=E6=96=87";

// If these elements combination exists in the code, an error will occur.
// But, even if at least one element is missing, it succeeds.
$headers = [
      "MIME-Version: 1.0"
    , "Content-Type: text/plain; charset=utf-8"
    , "Content-Transfer-Encoding: quoted-printable"
    , "" /* and empty string. */];
echo mail($sendto, $subject, $msg, implode("\r\n", $headers)) . "<br/>\n";
// -> "Warning: mb_send_mail(): Multiple or malformed newlines found in additional_header in xxx.php on line xx"

// Or, even if the newline-code is limited to LF, it will succeed.
echo mail($sendto, $subject, $msg, implode("\n", $headers)) . "<br/>\n";

Expected result:
----------------
Mail will be sent.


Actual result:
--------------
Mail was not sent.

An error occurred.
> "Warning: mb_send_mail(): Multiple or malformed newlines found in additional_header in xxx.php on line xx"