API でトピックを作成する際にタグを追加するためのタグIDの取得方法

Does anyone know if this should work in 2.0.0.beta9? Tags are not mentioned in the API docs for POST /posts.
I tried to create topics with tags="testtag" but got a HTTP 500 error.

From the logs:

NoMethodError (undefined method `-' for "testtag":String)                                                                                                                                                                                       
/var/www/discourse/lib/discourse_tagging.rb:220:in `tags_for_saving'