Allow to use associative arrays beside indexed arrays in wp_mail $headers

[marsjaninzmarsa]

eg:

$headers = [
	'From' => 'Me Myself <me@example.net>',
	'Cc'   => 'John Q Codex <jqc@wordpress.org>',
];

beside:

$headers = [
	'From: Me Myself <me@example.net>',
	'Cc: John Q Codex <jqc@wordpress.org>',
];

and also (because why not):

$headers = [
	'From' => 'Me Myself <me@example.net>',
	'Cc'   => [
		'John Q Codex <jqc@wordpress.org>',
		'iluvwp@wordpress.org',
	],
];

[marsjaninzmarsa]

It is also possible to use single line notation for multiple CC/BCC, eg:

$headers = [
	'From: Me Myself <me@example.net>',
	'Cc: Fo McFoo <foo@example.com>, bar@example.com',
];

[marsjaninzmarsa]

fixes (typo in previous filename)

[jorbin]

This looks like a good idea. It could use some unit tests though to be on the safe side. The current mail unit tests pass.

I've refreshed the patch above so that it applies cleanly.

[desrosj]

Looks like 30128.2.patch​ needs an additional refresh on trunk, and still needs unit tests.