Fix MIME-Version header field case

[zcalusic]

According to https://www.w3.org/Protocols/rfc1341/3_MIME-Version.html

"Messages composed in accordance with this document MUST include such a
header field, with the following verbatim text: MIME-Version: 1.0"

At least Thunderbird has been observed acting strangely when the field
is not properly cased.

[vodolaz095]

👍

I haven't encountered any bugs with this header having wrong case (by standart, headers are case insensitive).

But it's worth notice, that PHP Mailer (5.1 and 1.73 versions) and other MTA have this header in capital
like MIME-Version: 1.0

Message-ID: <[email protected]>
X-Priority: 3
X-Mailer: PHPMailer 5.1 (phpmailer.sourceforge.net)
Precedence: bulk
List-Unsubscribe: <https://www.telderi.ru/ru/system/unsubscribe/h/ad4d67629d407ad71149c9d19ee5a573>
List-id: <2017 December 06>
MIME-Version: 1.0
Content-Type: multipart/alternative;
	boundary="b1_e671070889360b45cb1d27b605cdccd3"

Subject: =?utf-8?B?0J/RgNC10LTQu9C+0LbQtdC90LjQtSDQv9C+0L/QvtC70L3QtdC90LjRjyA=?=  =?utf-8?B?0LDQu9GM0LHQvtC80LAg0L7RgiDQv9C+0LvRjNC30L7QstCw0YLQtdC70Y8g?=  =?utf-8?B?0JfQvtCx0LrQvtCyINCT0LXQvtGA0LPQuNC5ICh1cnphKQ==?=
X-PHP-Originating-Script: 1001:class.phpmailer.php
Date: Tue, 9 Jan 2018 18:19:35 +0300
From: =?utf-8?B?0KTQvtGC0L7QsdCw0L3QuiDQm9C+0YDQuA==?= <[email protected]>
Message-ID: <[email protected]>
X-Priority: 3
X-Mailer: PHPMailer [version 1.73]
MIME-Version: 1.0
Content-Type: multipart/alternative;
	boundary="b1_c0e708b764ee40c41e06ad3c78e98be1"

[vodolaz095]

@ivy - for me this changes looks viable

[ivy]

@zcalusic Thanks for this fix! I've merged your change in 2449c75.

@vodolaz095 Thanks for your help verifying/testing! 😄