Possible regression in df.copy() in 0.13
``` python
pd.DataFrame(index=[0])["a"] = 1 # this works
pd.DataFrame(index=[0]).copy()["a"] = 1 # this doesn't
```

The latter command raises:

```
pandas/core/internals.pyc in _add_new_block(self, item, value, loc)
   3040         new_block = make_block(value, self.items[loc:loc + 1].copy(),
   3041                                self.items, fastpath=True)
-> 3042         self.blocks.append(new_block)
   3043 
   3044         # set ref_locs based on the this new block
AttributeError: 'numpy.ndarray' object has no attribute 'append'
```

I'm using pandas 0.13. This used to work fine in 0.12
