BUG: SettingWithCopyWarning even if view
#### Code Sample, a copy-pastable example if possible

```python
# df['column_to_assign_to'][assigning_mask] = values_tobe_assigned

```
#### Problem description

the given code example raies a SettingWithCopyWarning but it's no copy but a view!

<details>
python: 3.6.2.final.0
pandas: 0.24.1
