BUG: astype assignment with loc not working
``` python
In [10]: df = pd.DataFrame({'A': [1., 2., 3., 4.]})

In [11]: df.dtypes
Out[11]: 
A    float64
dtype: object

In [12]: df.loc[:, 'A'] = df['A'].astype(np.int64)

In [13]: df.dtypes
Out[13]: 
A    float64
dtype: object
```
#4624 (which closed #4312) fixed it for `.iloc`
