Fix type cast exceptions from MongoDbClient
Prevent attempts to update _id field

Fixes MongoExceptions that were being raised previously
Merge pull request #1 from benjaminbrent/fix-mongodbclient-bug

This branch fixes two bugs in MongoDbClient:

ClassCastExceptions raised due to the incorrect handling of field values returned by database queries in read()
MongoExceptions raised due to attempts to update the _id field value in update()