[5.2] Make MessageBag constructor consistent with add method

[michaeldyrynda]

Currently, the behaviour between initialising a new MessageBag and using the add method is inconsistent.

The add method tests for uniqueness (of the key, and the message in the keyed bag), whereas the constructor will accept duplicate messages.

$messageBag = new MessageBag(['messages' => ['first', 'second', 'third', 'third']]);

// $messageBag->getMessages()
// ['messages' => ['first', 'second', 'third', 'third']]

$messageBag = new MessageBag;
$messageBag->add('messages', 'first');
$messageBag->add('messages', 'second');
$messageBag->add('messages', 'third');
$messageBag->add('messages', 'third');

// $messageBag->getMessages()
// ['messages' => ['first', 'second', 'third']

I'm not certain if this is on purpose, so I have opened a PR with a failing test to demonstrate the proposed behaviour, which will amend should the desire exist to match the behaviour between the constructor and public API of the MessageBag class.

It appears this has been around since January, 2013, so this would likely be targeted to next release due to backwards compatibility breakage.

[GrahamCampbell]

Will need to go to 5.3, yes.

[GrahamCampbell]

Please discuss proposals in the correct place as per the contribution guidelines.