Async Gorm - onComplete doesn't render JSON in a controller of the owner side

Using Grails 3.1.6, the owner side of an association (one-to-many, many-to-many.....) doesn't get JSON rendering when used in a method with Async Gorm in a Controller.

E.g.: Campus has many courses and a Course belongs to a Campus (bidirectional one-to-many relationship).
### Task List
- [X] Steps to reproduce provided
- [ ] Stacktrace (if present) provided
- [ ] Example that reproduces the problem uploaded to Github
- [X] Full description of the issue provided (see below)
### Steps to Reproduce
1. Create a bidirectional one-to-many relationship (E.g.: Campus has many Courses and Course belongs to Campus)
2. In Bootstrap and Controllers:

```
Campus.async.findAllByNomeIlike("%man%").onComplete{ results ->
println results.value //Bootstrap
render results.value //Controller
}

Course.async.findAllByNomeIlike("%tec%").onComplete{ results ->
println results.value //Bootstrap
render results.value //Controller
}
```
1. In Bootstrap and Controllers, this works:

```
Campus.async.findAllByNomeIlike("%$params.q%").onComplete{ results ->
render results.value as JSON //doesn't work
}

Course.async.findAllByNomeIlike("%$params.q%").onComplete{ results ->
render results.value as JSON //this works
}
```
### Expected Behaviour

Async JSON should be rendered at both sides. In CampusController, this shoukd work:

```
def getCampus() {
Campus.async.findAllByNomeIlike("%$params.q%").onComplete{ results ->
render results.value as JSON
}
}
```

But it doesn't.

In CourseController, this works:

```
def getCourses() {
Course.async.findAllByNomeIlike("%$params.q%").onComplete{ results ->
render results.value as JSON
}
}
```
### Actual Behaviour

The following happens after an unsuccessful "render results.value as JSON" on the owner side:

```

• An error has occurred


• Exception:


• Message:


• Path:

```
### Environment Information
- **Operating System**: Windows 7 (64-bit)
- **Grails Version:** 3.1.6
- **JDK Version:** Oracle 1.8.0.77
- **Container Version (If Applicable):** not applicable

Start by reproducing the issue in the Grails 3.1.6 setup described, using the bidirectional Campus/Course association and the two controller methods shown. Compare JSON rendering for the owner and inverse sides; done means both async controller queries render their association results as JSON without the reported error page.

Written by the indexing model from the issue text.