DataFrame stores complex number as object sometimes

``` python
>>> DataFrame([[0.0 + 1j, 0.0 + 2j]]).values
array([[1j, 2j]], dtype=object)
>>> DataFrame([np.array([1j, 2j])]).values
array([[ 0.+1.j,  0.+2.j]])
```

I think these two examples should yield the same result.
