[5.3] Add support for implicit keys in first() and has()

[themsaid]

In case the bag has the following messages:

['user.0.email' => 'email is required']

$messages->first('user.*.email') will return 'email is required'

and $messages->has('user.*.email') will return true.

Currently these two methods don't work with implicit keys, only get() currently works.

[taylorotwell]

Could you send this to 5.4?