Trumspringa

To reproduce open the playground in private browsing mode in Firefox.

Do Transactions work correctly with this library? I notice you only use the [:commit](https://cloud.google.com/firestore/docs/reference/rest/v1/projects.databases.documents/commit) endpoint but not[ :beginTransaction](https://cloud.google.com/firestore/docs/reference/rest/v1/projects.databases.documents/beginTransaction) or [:rollback](https://cloud.google.com/firestore/docs/reference/rest/v1/projects.databases.documents/rollback)?