Complex64 implicitly converted to Complex128

NumPy array of complex64 is implicitly converted to complex128 when used as data for DataFrame:

``` python
>>> a=np.random.randn(10).astype(np.complex64)
>>> a.dtype
dtype('complex64')
>>> pd.DataFrame({'a': a}).a.dtype
dtype('complex128')
>>> 
```

The problem does not affect Series:

``` python
In [7]: Series(a).dtype
Out[7]: dtype('complex64')
```
