When running inside a session and a transaction, `count_documents` doesn't include any changes that are in the current transaction as the session is not passed through from `Mongo::Collection::View`'s options to the `Aggregration` options.

https://github.com/mongodb/mongo-ruby-driver/blob/master/lib/mongo/collection/view/readable.rb#L214

should add `options[:session]` to `opts[:session]` - there are probably more options it would make sense to pass through. See also RUBY-1665