Creating DataFrame Changes Time Unit of datetime64 Series
On pandas v0.11.0, For example:

```
foo = pd.Series(np.array([np.datetime64(datetime.date(2012, 1, 1), 'M')]))
```

```
In [64]: foo.dtype
Out[64]: dtype('<M8[M]')
```

but then:

```
In [65]: pd.DataFrame({"date":foo,}).date.dtype
Out[65]: dtype('<M8[ns]')
```
