units: warning from numpy operation on Quantity arithmetic

The following gives a warning:

```
from astropy import units as u
import numpy as np

a = 15 * u.kpc
b = 27 * u.pc

print np.arcsin((b/a))
```

but doing this does not:

```
print np.arcsin((b/a).decompose())
```

The warning is:

```
RuntimeWarning: invalid value encountered in arcsin
  print np.arcsin((b/a))
```
