documentArray#id

Hi Aaron, I have this scnerio where I popuate the _id key of a document 
and this prevent the documentarray id method to work, this pull request solve that 
let me know what you think.

Here is the detailed scenario
```js

GameSchema = new Schema({
 players: [PlayerSchema]
 // other paths... 
})

PlayerSchema = new Schema(
 {_id: {type: ObjectId, ref: 'User'}}
 // other paths... 
}

Game
.findById(gameId)
.populate('players._id')
.exec(function(err, game)  {
  // game.players.id fails to find anything now  
  // because we populated players._id path
  // and game.players._id is a populated Document
  console.log game.players.id(playerId);
}) 
```