SkyCoord array does not work with float32

If I have a numpy.float32 array I cannot initiate a SkyCoords array, but get an error. I have to manually cast it to numpy.float64 first (when loading arrays from fits you often get float32 arrays).
Example:

```
from astropy import units
from astropy.coordinates import SkyCoord
import numpy as np

coords = SkyCoord(ra=np.float32([1,2])*units.degree, dec=np.float32([3,4])*units.degree, frame='icrs')
print coords
```

results in: "ValueError: new type not compatible with array."