User limits cannot be imported by the definitions file

[davidkloosterman]

User limits, introduced in 3.8.10, cannot be imported by the definitions file.

Steps to reproduce:

1. Create a definitions file with user limits. Tips for creation:
   a. Set up user limits by the console: rabbitmqctl set_user_limits user1 {"max-connections": 2}, or by the UI.
   b. Export definitions: rabbitmqctl export_definitions definitions_test.json, or by the UI
   c. Remove the users limits again by the the console or UI, i.e. step 1a will be reverted.
2. Load the new definitions file by the console: rabbitmqctl import_definitions definitions_test.json.
3. Check the new user limits by the the console (export definitions) or UI. Here you will see that the user limits are not set.