BUG: datetime64[ns] conversion to object in DataFrame constructor inconsistent
this seems to happend when there is an object type passed as well

```
In [55]: df = pd.DataFrame({'a':[1,2,4,7], 
    'b':[1.2, 2.3, 5.1, 6.3], 
    'c':list('abcd'), 
    'd':[pd.datetime(2000,1,1) for i in range(4)]})

In [56]: df
Out[56]: 
   a    b  c                    d
0  1  1.2  a  2000-01-01 00:00:00
1  2  2.3  b  2000-01-01 00:00:00
2  4  5.1  c  2000-01-01 00:00:00
3  7  6.3  d  2000-01-01 00:00:00

In [57]: df.dtypes
Out[57]: 
a      int64
b    float64
c     object
d     object

# this is ok
In [64]: df = pd.DataFrame({'a':[1,2,4,7], 'b':[1.2, 2.3, 5.1, 6.3], 'd':[pd.datetime(2000,1,1) for i in range(4)]})

In [65]: df.dtypes
Out[65]: 
a             int64
b           float64
d    datetime64[ns]

```
