Description:
------------
If imap_mail_compose() is used to create a multipart MIME message
without specifying explicit types for the individual parts, that
leads to a segfault.  Since the type for the first body defaults
to TYPETEXT, I would expect the individual parts to default to
TYPETEXT as well.


Test script:
---------------
<?php
$envelope = [
    'from' => 'me@example.com',
    'to' => 'you@example.com',
    'subject' => 'hello',
];
$body = [[
    'type' => TYPEMULTIPART,
], [
    'contents.data' => 'yada yada',
]];
echo imap_mail_compose($envelope, $body);
?>


Expected result:
----------------
From: me@example.com
Subject: hello
To: you@example.com
MIME-Version: 1.0
Content-Type: MULTIPART/MIXED; BOUNDARY="321-1709-1602347443=:9960"

--321-1709-1602347443=:9960
Content-Type: TEXT/PLAIN; CHARSET=US-ASCII

yada yada
--321-1709-1602347443=:9960--


Actual result:
--------------
segfault