REST API endpoint fails to validate payload properly

What's the issue?
At least one REST API endpoint does not validate requests' payloads correctly, returning response status code 200 even if the request has no effect.

Steps to reproduce:

1. Set up a one node cluster with data service enabled.
2. Create an empty couchbase bucket with a name "default".
3. Run

   curl -v --header "Content-Type: application/json" \

   --request POST \

   --data '{"flushEnabled":1}' \

   http://localhost:9000/pools/default/buckets/default \

   -u Administrator:asdasd

4. Observe that even though this request returns 200 status code, flush does not get enabled for the "default" bucket.