command {aggregate: 'test', pipeline: [{'$match' => {_id: true}}]} fails as the implementation checks for string 'pipeline' and not symbol :pipeline.

$ mongo_console
> Mongo::MongoClient.new.db('test').command({aggregate: 'test', pipeline: [{'$match' => {_id: true}}]})
NoMethodError: undefined method `none?' for nil:NilClass
from /Users/gjm/.rvm/gems/ruby-2.1.1/gems/mongo-1.11.1/lib/mongo/functional/read_preference.rb:77:in `secondary_ok?'