Description:
------------
Specifying a MIME-Version header when using mb_send_mail() results in such a header appearing twice in the e-mail message.

Also note that RFC2045 spells the initialism MIME in upper case and requires the text "MIME-Version: 1.0" to be verbatim (although ignoring RFC822 comment strings is required), see http://tools.ietf.org/html/rfc2045#section-4


Test script:
---------------
<?php
$headers = 'From: Someone <someone@somewhere.invalid>' . "\n"
    . 'MIME-Version: 1.0' . "\n"
    . 'Content-Type: text/plain; charset=UTF-8' . "\n"
    . 'Content-Transfer-Encoding: 8bit';
mb_send_mail('john@smith.invalid', 'Test', 'Testing', $headers);
?>


Expected result:
----------------
E-mail message with the following headers:

To: john@smith.invalid
Subject: Test
From: Someone <someone@somewhere.invalid>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Message-ID: <20100823191234.ABCDEF1234@host.domain.invalid>
Date: Mon, 23 Aug 2010 22:12:34 +0300 (EEST)


Actual result:
--------------
E-mail message has the following headers:

To: john@smith.invalid
Subject: Test
From: Someone <someone@somewhere.invalid>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Mime-Version: 1.0
Message-Id: <20100823191234.ABCDEF1234@host.domain.invalid>
Date: Mon, 23 Aug 2010 22:12:34 +0300 (EEST)