Fix sorting MIME children when their types are equal

[Grendel7]

Currently, if a message has two MIME children with the same content type, they will be added in their inverted order. In most cases, this won't matter, but for some e-mails (like X-ARF, which has two text/plain parts, one with human readable and one with YAML content), this is problematic.

This small fix ensures the original ordering is preserved when the typePrefs are identical.

[fabpot]

Thanks for the fix. Can you add a test for that behavior?

[Grendel7]

Sure, I will add a test for it ASAP.

[Grendel7]

After adding my test, it turns out that this fix causes a regression for PHP 5.

The reason for this is the ordering of the first and second argument of usort being different in PHP 5 and PHP 7.

Given a simple proof of content:

<?php
$array = ['one', 'two'];

usort($array, function($a, $b) {
	echo 'A: '.$a.' - B: '.$b;
	return 0;
});

The output for PHP 5.6 is:

A: two - B: one

But for PHP 7.0, the ouput is:

A: one - B: two

In other words, you can't make any assumptions about $a and $b regarding their position in the source array.

Because of that, I rewrote the entire sorting function to not rely on usort anymore. This implementation groups the mime parts first, sorts those and then flattens the sorted groups. This is the only method I could think of which reliably works on all PHP versions.

[fabpot]

Can you rebase? I've fixed Travis conf. Thanks

[Grendel7]

Thanks, I've rebased the branch and all tests are green now.

[fabpot]

Thank you @HansAdema.