Fix Identity field names

tests/reference.yaml

@@ -898,13 +898,13 @@ definitions:
         title: ID
         type: integer
         readOnly: true
-      firstName:
-        title: FirstName
+      first_name:
+        title: First name
         type: string
         maxLength: 30
         minLength: 1
-      lastName:
-        title: LastName
+      last_name:
+        title: Last name
         description: <strong>Here's some HTML!</strong>
         type: string
         maxLength: 30