For the PUT/POST calls right now default-operation tag in the edit-config is set to "none"  whose behavior is explained below as per RFC. 

For devices that enforces this behavior strictly when there is parent structure in the request, ( say for a nested model) , device will throw an error.

We should  leave this field out ( which will default to merge) and let the device make the right decision.
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.