Author extraction returns a String, not a Hash

* This commit modifies `AuthorExtraction` to return a String instead of
  a Hash by adding an `indexer` value. This behavior is consistent with
  `TitleExtraction`, which also extracts a single piece of metadata and
  returns a String.

* Additionally, this commit removes an unnecessary test that appears to
  have been inadvertantly copied and pasted from another spec file.

Closes #15