Check if key exists before iterating through dot-notation.

Allowing for array keys to contain "dots" to detect flattened id/value pairs (e.g. used by Symfony's Translation Component).
Fix some namespace/parameters in phpdocs
Merge pull request #646 from barryvdh/patch/phpdocs

Fix some namespace/parameters in phpdocs
Added array_fetch, array_flatten and Collection methods.
Added merge method to collection.
Tweak merge method and add tests.
Added "lists" to Eloquent so that mutators are run.
Make `add` and `merge` methods on the `MessageBag` chainable.
Added `pop` and `shift` methods to Eloquent collection.