Dataframe rename issue.
I just upgrage from 0.11 to 0.12 version. And meet  dataframe rename error caused by upgrading.  (It worked well in 0.11) .   

```
>>> df4
                 TClose      RT    TExg
STK_ID RPT_Date                        
600809 20130331   22.02  0.0454  0.0422

>>> df5
                 STK_ID  RPT_Date STK_Name  TClose
STK_ID RPT_Date                                   
600809 20120930  600809  20120930     山西汾酒   38.05
       20121231  600809  20121231     山西汾酒   41.66
       20130331  600809  20130331     山西汾酒   30.01

>>> k=pd.merge(df4, df5, how='inner', left_index=True, right_index=True)
>>> k
                 TClose_x      RT    TExg  STK_ID  RPT_Date STK_Name  TClose_y
STK_ID RPT_Date                                                               
600809 20130331     22.02  0.0454  0.0422  600809  20130331     山西汾酒     30.01

>>> k.rename(columns={'TClose_x':'TClose', 'TClose_y':'QT_Close'})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "d:\Python27\lib\site-packages\pandas\core\base.py", line 40, in __repr__
    return str(self)
  File "d:\Python27\lib\site-packages\pandas\core\base.py", line 20, in __str__
    return self.__bytes__()
  File "d:\Python27\lib\site-packages\pandas\core\base.py", line 32, in __bytes__
    return self.__unicode__().encode(encoding, 'replace')
  File "d:\Python27\lib\site-packages\pandas\core\frame.py", line 668, in __unicode__
    self.to_string(buf=buf)
  File "d:\Python27\lib\site-packages\pandas\core\frame.py", line 1556, in to_string
    formatter.to_string()
  File "d:\Python27\lib\site-packages\pandas\core\format.py", line 294, in to_string
    strcols = self._to_str_columns()
  File "d:\Python27\lib\site-packages\pandas\core\format.py", line 239, in _to_str_columns
    str_columns = self._get_formatted_column_labels()
  File "d:\Python27\lib\site-packages\pandas\core\format.py", line 435, in _get_formatted_column_labels
    dtypes = self.frame.dtypes
  File "d:\Python27\lib\site-packages\pandas\core\frame.py", line 1696, in dtypes
    return self.apply(lambda x: x.dtype)
  File "d:\Python27\lib\site-packages\pandas\core\frame.py", line 4416, in apply
    return self._apply_standard(f, axis)
  File "d:\Python27\lib\site-packages\pandas\core\frame.py", line 4491, in _apply_standard
    raise e
TypeError: ("'NoneType' object is not iterable", u'occurred at index TExg')

>>> df4.dtypes
TClose    float64
RT        float64
TExg      float64
dtype: object

>>> df5.dtypes
STK_ID       object
RPT_Date     object
STK_Name     object
TClose      float64
dtype: object
>>> 
```
