singleton lists are not rendered as lists by StructuredDataToJsonProvider

If I have a model with a list of string, like the leaf-list on line 32 here:
https://git.opendaylight.org/gerrit/#/c/8975/4/ttp-model/model/src/main/yang/ttp.yang

And I use the builder to add a singleton list of strings like this:
NDMMetadata NDMmeta = new NDMMetadataBuilder()
.setDoc(Arrays
.asList("TTP supporting L2 VLANs (unicast, multicast, flooding) with optional VID translation."))
.setAuthority("org.opennetworking.fawg").build();

When I covert it to JSON, I get a single string, not a list containing a single stiring. That is I get this:
{
"NDM_metadata":

{ "authority": "org.opennetworking.fawg", "doc": "TTP supporting L2 VLANs (unicast, multicast, flooding) with optional VID translation." }

}

but, I'd expect this:
{
"NDM_metadata":

{ "authority": "org.opennetworking.fawg", "doc": ["TTP supporting L2 VLANs (unicast, multicast, flooding) with optional VID translation."] }

}

Is this a bug or expected behavior?

If you want to check it out, this patch set has an example:
https://git.opendaylight.org/gerrit/#/c/8975/4/