Issue modifying data in object arrays in DataFrame
reported by user

```
import pandas
pandas
import numpy as np
from pandas import DataMatrix
Y = DataMatrix( np.random.random( (4,4) ) , index = ('a','b','c','d') , columns = ('e','f','g','h' ))
Y
Y['e'] = Y['e'].astype('object')
Y['g']['c'] = np.NaN
Y
Y.sum()
Y['g'].sum()
```

Appears to be caused by a stale reference to a block
