WIP: Add more unit tests for modeling

I will add more unit tests for modelling in the coming weeks.

* [x] Improve coverage for `constraints.py`
* [x] Improve coverage for `core.py`
* [ ] Improve coverage for `fitting.py`
* [x] Improve coverage for `functional_models.py`
* [x] Improve coverage for `parameters.py`
* [x] Improve coverage for `polynomial.py`
* [ ] Improve coverage for `projections.py`
* [x] Improve coverage for `rotations.py`
* [x] Improve coverage for `utils.py`
* [ ] I left a few TODOs in the code where I depend on other issues to be resolved ...

Further suggestions welcome.

To find the un-tested parts of `modeling` I use this command to generate the coverage report locally:
```python setup.py test --coverage```
There's also a nightly generated coverage report on the web:
https://jenkins.shiningpanda.com/astropy/job/astropy-coverage/coveragepy/?