Exisiting controller overridden by generated controller when using @Resource

### Task List

- [x] Steps to reproduce provided
- [x] Stacktrace (if present) provided
- [x] Example that reproduces the problem uploaded to Github
- [x] Full description of the issue provided (see below)

### Steps to Reproduce

Clone the repo here: https://github.com/beechovsky/test-rest-resource-controller

NOTE: This app was created using create-app, domain created using create-domain-class, and everything else using generate-all for the Student domain.

On the master branch, run the application and save a new Student (via the GUI). Check the terminal output and verify seeing the log message from hitting the save() and show() methods in the controller.

Next, checkout the with-jackson branch and run the app again.

Save a new instance (view is no longer accessible):
`curl -i -X POST -H "Content-Type: application/json" -d '{"firstName":"New","lastName":"Kid","age":21}' localhost:8080/students`

You should see the following or similar:

````
HTTP/1.1 201
Location: http://localhost:8080/students/3
Content-Type: application/json;charset=UTF-8
Transfer-Encoding: chunked
Date: Wed, 23 Dec 2020 14:34:01 GMT

{"id":3,"firstName":"New","age":21,"lastName":"Kid"}
````

Let’s also hit the ‘native’ existing endpoint from our _existing_ controller:

`curl -i -X POST -H "Content-Type: application/json" -d '{"firstName":"New","lastName":"Kid","age":21}' localhost:8080/student/save`

You should again see the following or similar:

```
HTTP/1.1 201
Location: http://localhost:8080/students/3
Content-Type: application/json;charset=UTF-8
Transfer-Encoding: chunked
Date: Wed, 23 Dec 2020 15:49:28 GMT

{"id":3,"firstName":"New","age":21,"lastName":"Kid"}
```

For both actions, review log output and see there is no indication we hit the save() or show() methods in the _existing_ controller despite a successful save.

In fact, simply navigate to localhost:8080/students/1 (and /student/show/1) and note the lack of logs from the controller at either endpoint.

Now, checkout the with-restful-controller branch and repeat the steps above.

Finally, check out with-ajax branch. NOTE: This branch most accurately mimics the real-world project.

Navigate to http://localhost:8080/ajax/ (create is the only action so it’s accessible here).

Click the ‘Save a Student’ button.

Return to the student list (it will be JSON) to verify the save worked. NOTE: The params are hard-coded for simplicity’s sake; if you need to save another entry just update the parameters in the Javascript in the create.gsp view for the AJAX controller.

### Expected Behaviour

The methods in the existing controller should be hit.

### Actual Behaviour

The methods in the existing controller are bypassed/overridden.

### Environment Information

- **Operating System(s)**: MacOS Catalina 10.15.7, RHEL 7, and Linux Mint 19 Tara
- **Grails Version(s):** 4.0.3, 4.0.5
- **JDK Version:** 1.8.0_242
- **Container Version (If Applicable):** N/A

### Example Application

- https://github.com/beechovsky/test-rest-resource-controller

Reproduce the behavior in the linked test-rest-resource-controller application, comparing the master, with-jackson, with-restful-controller, and with-ajax branches. Start with the /students, /student/save, /students/1, and /student/show/1 endpoints and inspect their logs; done means the existing controller methods are invoked rather than bypassed or overridden.

Written by the indexing model from the issue text.