BUG: Fix groupby with "as_index" for categorical multi #13204
- [x] closes #13204
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Fixes a bug that returns all nan's for groupby(as_index=False) with
multiple column groupers containing a categorical one (#13204).

---

Also:
fixes an internal bug in the string representation of `Grouping`.

``` python
mi = pd.MultiIndex.from_arrays([list("AAB"), list("aba")])
df = pd.DataFrame([[1,2,3]], columns=mi)
gr = df.groupby(df[('A', 'a')])

gr.grouper.groupings
...
TypeError: not all arguments converted during string formatting
```
