"Unauthorized User" error occures when retrieving a collection with a 'dbOwner' user.
The following code runs correctly, if an admin user is specified in Mongo::Client.new option,

1. Ruby Code
   require 'mongo'
   > db = Mongo::Client.new(
   ['test-server:20001'],
   :connect => :sharded,
   :database => "test",
   :user => "test_user", # run correctly if :user => "admin_user"
   :password => "test_pass")
   > db["test_col"].find.count

Mongo::Auth::Unauthorized: User test_user is not authorized to access test.
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/auth/scram/conversation.rb:445:in `validate!'
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/auth/scram/conversation.rb:440:in `validate_first_message!'
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/auth/scram/conversation.rb:112:in `continue'
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/auth/scram.rb:45:in `login'
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/server/connection.rb:64:in `connect!'
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/server/connectable.rb:89:in `ensure_connected'
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/server/connection.rb:130:in `write'
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/server/connectable.rb:66:in `block in dispatch'
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/loggable.rb:44:in `log'
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/loggable.rb:65:in `log_debug'
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/server/connectable.rb:65:in `dispatch'
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/operation/command.rb:51:in `block in execute'
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/server/connection_pool.rb:99:in `with_connection'
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/server/context.rb:62:in `with_connection'
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/operation/command.rb:50:in `execute'
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/database.rb:136:in `command'
from /home/dep/.rvm/gems/ruby-2.1.2/gems/mongo-2.0.2/lib/mongo/collection/view/readable.rb:124:in `count'
: