pct_change() fails on series holding int64

``` python
In [132]: df
Out[132]: 
    A   B
a  10   8
b  12  16
c  12  24

In [133]: df.dtypes
Out[133]: 
A    int64
B    int64

In [134]: df.pct_change()
Out[134]: 
     A    B
a  NaN  NaN
b  0.2  1.0
c  0.0  0.5

In [135]: df['A'].pct_change()
---------------------------------------------------------------------------
...
ValueError: Invalid dtype for padding
```
