Fix ACursor.get to decode failures as Option

The following two ways of decoding a field's value were not equivalent:

  1. downField(a).downField(b).downField(c).as[Option[A]]
  2. downField(a).downField(b).get[Option[A]](c)

More details in the tests.
Merge pull request #75 from igstan/acursor-get-option

Fix ACursor.get to decode failures as Option