groupProperty don't work on Unit Test

### Task List

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

### Steps to Reproduce

1. Create a domain class with a group field
2. Insert some data to test
3. Create a Unit Test to check the group by

### Expected Behaviour
The "group by" statement should return a group result
```groovy
new User(name: 'user1', team: 'team1').save(flush: true)
new User(name: 'user2', team: 'team2').save(flush: true)
new User(name: 'user3', team: 'team2').save(flush: true)

def result = User.createCriteria().list {
projections {
groupProperty 'team'
}
}

result.size() == 2
result == [team1, team2]
```

### Actual Behaviour
The "group by" statement return a wrong result
```groovy
new User(name: 'user1', team: 'team1').save(flush: true)
new User(name: 'user2', team: 'team2').save(flush: true)
new User(name: 'user3', team: 'team2').save(flush: true)

def result = User.createCriteria().list {
projections {
groupProperty 'team'
}
}

result.size() == 3 // must be 2
result == [team1, team2, team2]
```
### Environment Information

- **Operating System**: Ubuntu 14.04.1 x86_64
- **GORM Version:** 6.1.6.RELEASE
- **Grails Version (if using Grails):** 3.3.0
- **JDK Version:** 1.8.0_40

### Example Application

[Grails Application with Unit Test to play](https://github.com/mcaligares/gorm-test)

Start with the linked example application and its unit test, reproducing the User.createCriteria() query with projections and groupProperty 'team'. Compare the returned values with the expected two distinct teams; the issue is done when the test produces two grouped results rather than three rows.

Written by the indexing model from the issue text.