Aviat\Ion\Config
  * Added `has($key)` method to check if a config item exists

Aviat\Ion\Type\ArrayType
  * Updated `hasKey($key)` method to accept an array key, so it matches
  the get and set methods

Aviat\Ion\Transformer\AbstractTransformer
  * Added `untransformCollection($collection)` method, which uses the
  `untransform($item)` method on the extended class. It throws a
  `BadMethodCallException` if the `untransform($item)` method is not
  defined.