Updating patient `nonstandard_features` via REST may clear other features as well

PUT

also deletes all regular, standard features

E.g. create a patient with some features, suppose it is patient `P0000001`. Then try

curl -u Admin:admin -H "Content-Type: application/json" -X PUT -d '{"nonstandard_features": []}' http://localhost:4444/rest/patients/P0000001

and all the features will be removed. Other items are not affected and setting other parameters works, e.g. if sex is set to "M" it will stay as "M", and a call like

curl -u Admin:admin -H "Content-Type: application/json" -X PUT -d '{"nonstandard_features": [], , "sex":"F"}' http://localhost:4444/rest/patients/P0000001

will correctly update the sex but also blank regular features