Error "buffer source array is read-only" when using unique() on df loaded with feather
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import feather

df1 = pd.DataFrame([1,2,3,1,2,4], columns=['a'])
df1.a = df1.a.astype('category')
print(df1.a.unique())

df1.to_feather('a.feather')
df1 = feather.read_dataframe('a.feather')
df1.a.unique()
```
