Enable the ElasticSearch DocManager to handle dbrefs

I’m using the connector on my project, and it was failing because many of my documents include DBrefs. This change enables these documents to be added to the ElasticSearch indices successfully.

I included a test, but I wasn’t able to actually confirm that the test works, as I had trouble running the tests. I would appreciate it if someone else could try the test and see if it works. I’m not too worried about it, though, as I’ve tested the actual code in my actual project, and it works.