Merge DataFrame and Series using `on`
Now that we support merging `on` combination of column names and index levels, this should work

```python
In [40]: a = pd.DataFrame({"A": [1, 2, 3, 4]}, index=pd.MultiIndex.from_product([['a', 'b'], [0, 1]], names=['outer', 'inner']))

In [41]: b = pd.Series([1, 2, 3, 4], index=pd.MultiIndex.from_product([['a', 'b'], [1, 2]], names=['outer', 'inner']), name='B')

In [42]: pd.merge(a, b, on=['outer', 'inner'])
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-42-337c5a9e9f8f> in <module>()
----> 1 pd.merge(a, b, on=['outer', 'inner'])

~/Envs/dask-dev/lib/python3.6/site-packages/pandas/core/reshape/merge.py in merge(left, right, how, on, left_on, right_on, left_index, right_index, sort, suffixes, copy, indicator, validate)
     58                          right_index=right_index, sort=sort, suffixes=suffixes,
     59                          copy=copy, indicator=indicator,
---> 60                          validate=validate)
     61     return op.get_result()
     62

~/Envs/dask-dev/lib/python3.6/site-packages/pandas/core/reshape/merge.py in __init__(self, left, right, how, on, left_on, right_on, axis, left_index, right_index, sort, suffixes, copy, indicator, validate)
    524         if not isinstance(right, DataFrame):
    525             raise ValueError('can not merge DataFrame with instance of '
--> 526                              'type {right}'.format(right=type(right)))
    527
    528         if not is_bool(left_index):

ValueError: can not merge DataFrame with instance of type <class 'pandas.core.series.Series'>
```

Should be the same as

```python
In [39]: pd.merge(a, b.to_frame(), on=['outer', 'inner'])
Out[39]:
             A  B
outer inner
a     1      2  1
b     1      4  3
```