minor: fixing typo in constant comment
RUBY-690 Test that aggregate supports explain option
Don't merge in named nil opts into command selector
RUBY-669 Support aggregation out pipeline operator
Check for the existance of a key instead of checking key value
Cleaning up references to system.users and unnecessary auth test
Extra options are merged into the selector so ensure OrderedHash
OrderedHash needed for commands with Ruby 1.8.7

RUBY-676
RUBY-686 using set for auths to fix issues with logout

- Switching MongoClient#auths to be a Set rather than an Array (still
Enumerable) so that we can fix an issue with logout and be a little more
intelligent when applying auths to a given socket.

- Consolidated requires for 'set' and 'thread'