If I set an `:op_timeout` value and then I firewall the database, then first call to `MongoClient.active?` raises an error. IMO it makes sense for this method to never throw but return `false`.

[84] pry(#<Object>)> db.active?
Mongo::OperationTimeout: Timed out waiting on socket read.
from /usr/local/share/gems/gems/mongo-1.9.2/lib/mongo/networking.rb:321:in `rescue in receive_message_on_socket'

Also I think that this method should not block indefinitely if database is firewalled/inaccessible and `:op_timeout` is `nil`. Otherwise this method becomes useless for any practical purposes. In my opinion it should just return true or false whenever database is accessible or not accessible.