to_csv drops values if DataFrame is int64 but assigned array of float64
If I assign a subset of a DataFrame column of type int64 an array of values of float64, printing the DataFrame's values will show the result of the assignment but dumping it to csv will not.

Minimal Reproducible Example on my system (Ubuntu 13.04, python 2.7.4, pandas 0.13.1, numpy 1.8.0):

``` python
import pandas as pd

df = pd.DataFrame({'target':np.zeros(10)}, dtype='int64')
df['target'][0] = 3333.0
df['target'][0:2] = [9999.0, 9999.0]
print df['target']
df.to_csv('bug.csv', index=False)
```

Printing df:

```
0    9999
1    9999
Name: target, dtype: int64
```

But the csv file is:

```
target
3333
0
```
