Alises to functions fail since version 0.11.0

For versions <= 0.10.0 it was possible to create an alias to a function. The regression was introduced by https://github.com/firstandthird/load-grunt-config/commit/665ce854afa48540f11e0ff49872af35fd0e1760#diff-d41d8cd98f00b204e9800998ecf8427e

To reproduce, create an alias to a function:

aliases.coffee:
```coffee
module.exports =
    someAlias: () -> 'Do something'
```
If you run that alias with grunt, the following message appears:
```

Running "someAlias" task
Warning: Task "undefined" not found. Use --force to continue.
````

This PR fixes that regression, and adds a unit test for it.
The unit test needs to overwrite the `loadGruntConfig` mock set at `setup` to avoid more changes in other tests.