Start service without target to keep playing on current device

[GitHubGoody]

If I do not specify a target device/entity (device_name, spotify_device_id, or entity_id) in a spotcast.start service call, I get a "Either entity_id or device_name must be specified" error. I know it is working as designed, but I would like to be able to default to the device currently playing on the default/specificed account with the new playlist.

For reference, Mini Media Player shortcuts can start a playlist without reference to the target entity.