BUG: setting object dtype Series row with an Extension Array
On 1.2 master:
``` python
In [11]: import numpy as np
    ...: import pandas as pd
    ...: 
    ...: ser = pd.Series(1, index=list("abcde"), dtype="object")
    ...: 
    ...: expected = pd.array([0,0,3,0,0])
    ...: ser.loc["a"] = expected
    ...: result = ser[0]

In [12]: ser
Out[12]: 
a    0
b    0
c    3
d    0
e    0
dtype: Int64
```
I expect:
``` python
In [14]: ser
Out[14]: 
a    [0, 0, 3, 0, 0]
b               1
c               1
d               1
e               1
dtype: object
```