Allow duplicate index values for to_csv
``` python
In [22]: df
Out[22]:
              0         1         2
A 1 2  0.659855  0.340490  0.351028
    2  0.695581  0.264714  0.389929
B   2  0.105016  0.070774  0.114629

In [23]: df.to_csv('foo.csv')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)

TypeError: only integer arrays with one element can be converted to an index

In [24]: df1
Out[24]:
          0         1         2
a  0.313015  0.631747  0.091818
a  0.505238  0.260553  0.793378
b  0.441791  0.029851  0.139490

In [25]: df1.to_csv('foo.csv')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
```
