BUG?: `_is_view == False` for DataFrame from DataFrame
Constructing off a dataframe creates a _de facto_ view, but `._is_view` reports false. Seems like a bug. 

```
In [1]:
    df = pd.DataFrame({'col1':[1,2], 'col2':[3,4]})
    df2 = pd.DataFrame(df)
    df2._is_view     
Out[1]:
    False

In [2]:
    df.loc[0,'col1'] = -88
    df2
Out[2]:
    col1    col2
    0   -88 3
    1   2   4
```

Suggestions on how best to fix?
