NETCONF's default-operation of "none" prevents config creation

I'm not sure how we haven't noticed this before. Or maybe we have and decided not to fix it?

ODL NETCONF sets default-operation to "none" southbound when using RESTCONF PUT or POST to modify a mounted device.

that's fine for e.g. adding a new item to a list as config already exists at that level.

but when adding new config it fails - correctly as far as I can tell from reading the relevant section of RFC6241:

none: The target datastore is unaffected by the configuration
in the <config> parameter, unless and until the incoming
configuration data uses the "operation" attribute to request
a different operation. If the configuration in the <config>
parameter contains data for which there is not a
corresponding level in the target datastore, an <rpc-error>
is returned with an <error-tag> value of data-missing.
Using "none" allows operations like "delete" to avoid
unintentionally creating the parent hierarchy of the element
to be deleted.

So in this case I'm getting rpc-error from the mounted device with an error-tag value of data missing...