Removing optional properties in PATCH requests?

[garethsb]

Related to #46...

E.g. can a client PATCH an Endpoint or a Network Flow to remove their "label" or other optional properties (e.g. Endpoint "max_bandwidth")?

This hasn't come up in e.g. IS-05 because there are no optional "transport_params", they just may support null.

We could support patching null to remove an optional property, because optional properties that also allow null are unlikely. This would also actually be more in line with RFC 7386 JSON Merge Patch.

[garethsb]

Another option is to do this in v1.1 by making these properties required but allowing null.