Assignment to mixed-type DataFrame with float32 coerce to float64
Assignment to mixed-type DataFrame with float32 numpy array coerce to float64

```
>>> import pandas as pd
>>> import numpy as np
>>> df = pd.DataFrame([[1, 2, 'a'], [3, 4, 'b']], dtype=np.float32)
>>> A = df.ix[:, 0:1].values
>>> df.ix[:, 0:1] = A
>>> df[0].dtype
dtype('float64')
```

Shouldn't the result be `dtype('float32')`?

See also http://stackoverflow.com/questions/35230388/why-does-pandas-coerce-my-numpy-float32-to-float64
