Don't pass ruby-formatted opts through to createIndex command

This failure made me aware of an issue in our create_index method : 
https://jenkins.10gen.com/view/Ruby/job/mongo-ruby-driver-emilys-fork/59/mongodb_server=master-nightly-release,os_arch=linux64,ruby_language_version=2.0.0,test_type=without-ext/console

The drop_dups and bucket_size options were not being deleted from the opts passed through to the createIndex command.