pd.merge fails if columns of only one side are hierarchical (even if index is equal)
```
import pandas as pd
import numpy as np
df = pd.DataFrame([(1,2,3), (4,5,6)], columns = ['a','b','c'])
new_df = df.groupby(['a']).agg({'b': [np.mean, np.sum]})
other_df = df = pd.DataFrame([(1,2,3), (7,10,6)], columns = ['a','b','d'])
other_df.set_index('a', inplace=True)
print new_df
print other_df
pd.merge(new_df, other_df, left_index=True, right_index=True)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-11-4c9da13e85ff> in <module>()
      7 print new_df
      8 print other_df
----> 9 pd.merge(new_df, other_df, left_index=True, right_index=True)

/usr/local/lib/python2.7/dist-packages/pandas-0.9.0rc2-py2.7-linux-x86_64.egg/pandas/tools/merge.pyc in merge(left, right, how, on, left_on, right_on, left_index, right_index, sort, suffixes, copy)
     31                          right_index=right_index, sort=sort, suffixes=suffixes,
     32                          copy=copy)
---> 33     return op.get_result()
     34 if __debug__: merge.__doc__ = _merge_doc % '\nleft : DataFrame'
     35 

/usr/local/lib/python2.7/dist-packages/pandas-0.9.0rc2-py2.7-linux-x86_64.egg/pandas/tools/merge.pyc in get_result(self)
    181 
    182         # this is a bit kludgy
--> 183         ldata, rdata = self._get_merge_data()
    184 
    185         # TODO: more efficiently handle group keys to avoid extra consolidation!

/usr/local/lib/python2.7/dist-packages/pandas-0.9.0rc2-py2.7-linux-x86_64.egg/pandas/tools/merge.pyc in _get_merge_data(self)
    271         lsuf, rsuf = self.suffixes
    272         ldata, rdata = ldata._maybe_rename_join(rdata, lsuf, rsuf,
--> 273                                                 copydata=False)
    274         return ldata, rdata
    275 

/usr/local/lib/python2.7/dist-packages/pandas-0.9.0rc2-py2.7-linux-x86_64.egg/pandas/core/internals.pyc in _maybe_rename_join(self, other, lsuffix, rsuffix, copydata)
   1115 
   1116     def _maybe_rename_join(self, other, lsuffix, rsuffix, copydata=True):
-> 1117         to_rename = self.items.intersection(other.items)
   1118         if len(to_rename) > 0:
   1119             if not lsuffix and not rsuffix:

/usr/local/lib/python2.7/dist-packages/pandas-0.9.0rc2-py2.7-linux-x86_64.egg/pandas/core/index.pyc in intersection(self, other)
   2270         Index
   2271         """
-> 2272         self._assert_can_do_setop(other)
   2273 
   2274         if self.equals(other):

/usr/local/lib/python2.7/dist-packages/pandas-0.9.0rc2-py2.7-linux-x86_64.egg/pandas/core/index.pyc in _assert_can_do_setop(self, other)
   2319             if len(other) == 0:
   2320                 return True
-> 2321             raise TypeError('can only call with other hierarchical '
   2322                             'index objects')
   2323 

TypeError: can only call with other hierarchical index objects

      b     
   mean  sum
a           
1     2    2
4     5    5
    b  d
a       
1   2  3
7  10  6
```

also this fails

```
pd.merge(new_df.reset_index(), other_df.reset_index(), left_index=False, right_index=False, left_on =('a', ''), right_on ='a' )
```

and this

```
pd.merge(new_df.reset_index(), other_df.reset_index(), left_index=False, right_index=False, left_on =[('a', '')], right_on =['a'] )
```
