Series does not upconvert datetimes to datetime64
``` python
import pandas as pd
ind = pd.date_range(start="2000-01-01", freq="D", periods=10)
datetimes = [ts.to_pydatetime() for ts in ind]

datetime_s = pd.Series(datetimes)
datetime_s.dtype
# object
df = pd.DataFrame({'datetime_s':datetime_s})
print df.dtypes
#datetime64
```

Shouldn't Series upconvert to datetime64 since the DataFrame columns do?
