RUBY-686 mongoclient changes for auth refactor and light cleanup

* Using a Set instead of Array for MongoClient#auths
* Behavior correction: default DB is now 'admin' (same as all other drivers)
* Moved an undocumented internal method to private
* A little light style/format cleanup (if you touch it make it better!)
RUBY-686 test coverage for auth mechanism and auth source support

* additional tests to cover URIParser support for authSource and authMechanism
* updated existing tests for the related changes