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
batch_write_incremental / old API completed

RUBY-676
Extra options are merged into the selector so ensure OrderedHash
Merge branch '1.x-stable' into 1.x-stable-ruby-676