Angle.to_string() returns a 0-d array with dtype=object

Not sure if this is intended:
```
In [1]: from astropy.coordinates.angles import Angle
In [2]: a = Angle('45d')
In [3]: a.to_string()
Out[3]: array(u'45d00m00.00000s', dtype=object)
```
I would have expected a plain string (or unicode obj) here.

In the case where the `Angle` is representing an array of angles, I would expect `to_string()` to return a numpy array with `dtype='S..'`.