Action null does not match an interceptor index action in Grails 3.2.x

We are porting a pretty large web app from Grails 2.4.5 to Grails 3.2.7.

We had to convert all filters and `beforeInterceptor` in controllers to real interceptors.

We observed a strange behavior and consequently asked about it on

[] (https://stackoverflow.com/questions/46710640/how-to-match-action-null-for-a-controller-in-grails-3-2-x-interceptor)

There it was suggested to open a bug report.

### Steps to Reproduce

1. We have a controller with some actions, 'index|details|prizeBreakdown', (DrawController)
2. We created an interceptor with `match(controller: 'draws', action: 'index|details|prizeBreakdown')`
3. We attempt to intercept the `boolean before()` aspect.
4. We do a `GET` to URL `/draws`

### Expected Behaviour

`before()` should be executed like in Grails 2 filters.

### Actual Behaviour

`before()` is never executed because `action` is `null` and it does not match `index` action.

### Environment Information

- **Operating System**: Ubuntu 16.04.3 LTS, x86-64
- **Grails Version:** 3.2.7
- **JDK Version:** 1.8.0_144-b01, x86-64
- **Container Version (If Applicable):** N/A

Start with the DrawController actions and the interceptor's match(controller: 'draws', action: 'index|details|prizeBreakdown') configuration. Reproduce the GET request to /draws on Grails 3.2.7 and trace why action is null; done means before() executes for that URL as it did with the Grails 2 filters.

Written by the indexing model from the issue text.