BUG: DataFrame.groupby() interprets tuple as list of keys 
- [x] closes #17979 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

```
In [2]: df = pd.DataFrame([[1, 2, 3, 4], [3, 4, 5, 6], [1, 4, 2, 3]],
   ...:                            columns=pd.MultiIndex.from_arrays([['a', 'b', 'b', 'c'],
   ...:                                                               [1, 1, 2, 2]]))

In [3]: df.groupby([('b', 1)]).groups
Out[3]: {2: Int64Index([0], dtype='int64'), 4: Int64Index([1, 2], dtype='int64')}

In [4]: df.groupby(('b', 1)).groups
Out[4]: {2: Int64Index([0], dtype='int64'), 4: Int64Index([1, 2], dtype='int64')}
```