fix(rbac) include Negative when marshaling RBAC

Include the Negative field when marshaling RBAC types to JSON.

The RBAC entity and endpoint permissions types have custom JSON
marshalers to account for differences between Kong's GET output and the
expected inputs to PUT/PATCH/POST:
https://github.com/Kong/go-kong/pull/5/files#r495600736

Previously, these marshalers omitted the Negative fields, so requests
outbound from go-kong would never set Negative, and it would always be
the default (false).

Fix FTI-2405

kong/types.go

@@ -363,6 +363,7 @@ func (e *RBACEndpointPermission) MarshalJSON() ([]byte, error) {
 		Workspace: e.Workspace,
 		Endpoint:  e.Endpoint,
 		Actions:   String(strings.Join(actions, ",")),
+		Negative:  e.Negative,
 		Comment:   e.Comment,
 	})
 }
@@ -399,6 +400,7 @@ func (e *RBACEntityPermission) MarshalJSON() ([]byte, error) {
 		EntityID:   e.EntityID,
 		EntityType: e.EntityType,
 		Actions:    String(strings.Join(actions, ",")),
+		Negative:   e.Negative,
 		Comment:    e.Comment,
 	})
 }