Issue title: Authenticated user 
 Issue body: Using the Facebook oath2 settings I've managed to connect and authenticate with Facebook, but every time I authenticate the same user is added to the DB (using NeDB), with the same id, _id and facebookId — over and over.

As if it's not checking if the user exists.

The app was generated with the feathers generate command, and authentication and the like added using the feathers book documentation. 
 Comments: 
 Comment 0: This is the normal behavior of NeDB.  Here's an example: https://github.com/louischatriot/nedb/issues/410.  See the section on persistence for how to clean it up.  I originally programmed the adapter to automatically compact every 5 seconds, but I'm not sure what it is setup to do right now.
 Comment 1: Please open an issue for the NeDB adapter.  This isn't a problem with the auth plugin.
 Comment 2: And the issue appears to be that the datafile isn't getting compacted often enough.
{'source': 'GithubIssues', 'repo_name_with_owner': 'feathersjs/feathers-authentication', 'issue_id': 152575114, 'repo_stars': 223.0}