Description of Drivers Ticket:

Relevant change to connection string spec:

If a driver allows URI options to be specified outside of the connection string (e.g. dictionary parameter to the MongoClient constructor) it MUST document the precedence rules between all such mechanisms. For instance, a driver MAY allow a value for option foo in a dictionary parameter to override the value of foo in the connection string (or vice versa) so long as that behavior is documented.

See DRIVERS-616 for updated details.