title: Can connect to MongoDB in Heroku through Mongo shell but not application
body: <p>I have setup a scala play reactive-mongo application in heroku. Mongo is provided by mlab in heroku. Having a bit of an issue connecting to mongo through the app, connecting through the shell with the same credentials works fine.  I am using scala play 2.5.1 and reactive 0.11.11 and mongod is 3.0.8.  I think all these versions work together but I get an authentication error at startup:    </p>
  
  <pre><code>2016-04-18T06:59:54.357736+00:00 app[web.1]: [info] application - ReactiveMongoApi starting...
  2016-04-18T06:59:54.586309+00:00 app[web.1]: [info] application - ReactiveMongoApi successfully configured with DB 'heroku_0pps6l5f'! Servers:
  2016-04-18T06:59:54.586332+00:00 app[web.1]:        [ds051635.mlab.com:51635]
  2016-04-18T06:59:55.101238+00:00 heroku[router]: at=info method=POST path="XXXXX" host=YYYYYY request_id=ec31a3df-88a1-4dc4-b195-5d39280db41b fwd="90.200.12.189" dyno=web.1 connect=0ms service=2382ms status=200 bytes=137
  2016-04-18T06:59:56.010123+00:00 app[web.1]: [error] r.c.a.MongoDBSystem - authentication failure
  2016-04-18T06:59:56.010133+00:00 app[web.1]: reactivemongo.core.commands.FailedAuthentication: BSONCommandError['auth failed'] with original doc {
  2016-04-18T06:59:56.010134+00:00 app[web.1]:   ok: BSONDouble(0.0),
  2016-04-18T06:59:56.010135+00:00 app[web.1]:   errmsg: "auth failed",
  2016-04-18T06:59:56.010135+00:00 app[web.1]:   code: BSONInteger(18)
  2016-04-18T06:59:56.010136+00:00 app[web.1]: }
  </code></pre>
  
  <p>On subsequent inserts I don't get an error but nothing actually gets inserted.<br>
  On retrievals I get the following error:</p>
  
  <pre><code>2016-04-18T07:05:56.742757+00:00 app[web.1]: Caused by: java.lang.RuntimeException: (,List(ValidationError(List('n' is undefined on object: {"ok":0,"errmsg":"not authorized on heroku_0pps6l5f to execute command { count: \"sightings\" }","code":13}),WrappedArray())))
  </code></pre>