Returning the string representation instead of the instance
variables of the object.

    Before:

    ```ruby
    IPAddr.new("127.0.0.1").to_json
    # => "{\"addr\":2130706433,\"family\":2,\"mask_addr\":4294967295}"
    ```

    After:

    ```ruby
    IPAddr.new("127.0.0.1").to_json
    # => "\"127.0.0.1\""
    ```

Fixes #40932.