Scaling replication controllers via API in Kubernetes
After long playing with it and consulting on slack DevOps chat, I was able to figure it out. Here is a workable curl: 

curl -sSk -H \"Authorization: Bearer ${KUBE_TOKEN}\" -H 'Content-Type: application/merge-patch+json' -X 'PATCH' $URL_NAME/api/v1/namespaces/kube-system/replicationcontrollers/kube-registry-v0 -d '{"spec": {"replicas":  1}}'


One of the important components here is "Content-Type: application/merge-patch+json" Without it specification, Kubernetes will not recognize data in API request correctly.