Metadata API: Targets.delegations should be optional #1362
Description
The delegation field in all targets type metadata seems to be optional by specification (see current reference implementation and theupdateframework/specification#157)
This is a potentially breaking change for a client but fortunately no-one's built one with Metadata API yet.
Current behavior:
In Metadata API deserializing target metadata currently fails if delegations field is missing.
The Target constructor argument is not optional.
Expected behavior:
"delegations" field should not be required in Metadata API deserialization and. delegations=None should lead to serialized format that does not include the field "delegations".