Casting float column to int does not work
```

In [1]: df = DataFrame(np.random.randint(0, 5, 10), dtype=float)

In [2]: df
Out[2]: 
   0
0  1
1  4
2  0
3  0
4  1
5  1
6  2
7  4
8  3
9  3


In [3]: df[0] = df[0].astype('i8')

In [4]: df
Out[4]: 
   0
0  1
1  4
2  0
3  0
4  1
5  1
6  2
7  4
8  3
9  3


In [5]: df.dtypes
Out[5]: 
0    float64
Name: None, Length: 1
```
