This change loosens the constraints on including attributes in
Locate messages for KMIP 2.0 clients. An early version of the KMIP
2.0 specification made attributes a required field for the Locate
payloads, breaking backwards compatibility with earlier versions
of the specification. A newer version of KMIP 2.0 reverses this
change. The library has been updated to reflect this newer version
of the specification. All relevant Locate unit tests have been
modified or removed to reflect this change.
Fixes#556