BUG/API: pivot_table with multi-index columns only
#### Code Sample, a copy-pastable example if possible

```python
In [21]: df = pd.DataFrame({'k': [1, 2, 3], 'v': [4, 5, 6]})

In [22]: df.pivot_table(values='v', columns='k')
Out[22]: 
k  1  2  3
v  4  5  6

In [23]: df.pivot_table(values='v', index='k')
Out[23]: 
   v
k   
1  4
2  5
3  6

In [24]: df2 = pd.DataFrame({'k1': [1, 2, 3], 'k2': [1, 2, 3], 'v': [4, 5, 6]})

In [25]: df2.pivot_table(values='v', index=('k1','k2'))
Out[25]: 
       v
k1 k2   
1  1   4
2  2   5
3  3   6

In [26]: df2.pivot_table(values='v', columns=('k1','k2'))
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-26-80d7fdeb9743> in <module>()
----> 1 df2.pivot_table(values='v', columns=('k1','k2'))

~\Anaconda\envs\py36\lib\site-packages\pandas\core\reshape\pivot.py in pivot_table(data, values, index, columns, aggfunc, fill_value, margins, dropna, margins_name)
    172     # discard the top level
    173     if values_passed and not values_multi and not table.empty and \
--> 174        (table.columns.nlevels > 1):
    175         table = table[values[0]]
    176 

~\Anaconda\envs\py36\lib\site-packages\pandas\core\generic.py in __getattr__(self, name)
   3075         if (name in self._internal_names_set or name in self._metadata or
   3076                 name in self._accessors):
-> 3077             return object.__getattribute__(self, name)
   3078         else:
   3079             if name in self._info_axis:

AttributeError: 'Series' object has no attribute 'columns'
```
#### Expected Output
No error, symmetrical between rows/columns and single/multi case

#### Output of ``pd.show_versions()``
`pandas 0.20.2`