Fix bulkIndex to work with documents that have _id

bulkIndex fails for documents that have _id.
This PR fixes that by looking at each document's _id (if exists) and assigning them to the configuration object pushed into the array. 

(note: I actually implemented the same thing in my code in coffeescript and it works well. I then backported to this PR in JS, so I never actually ran the JS code as is. But it's a simple fix...)