Fix formatting of Angle objects when precision=0

Before:

```

In [4]: from astropy.coordinates import Angle

In [5]: from astropy import units as u

In [6]: Angle(3., unit=u.deg).to_string(precision=0)
Out[6]: array(u'3d00m000s', dtype=object)
```

(notice the three digits for the seconds)

After:

```
In [4]: from astropy.coordinates import Angle

In [5]: from astropy import units as u

In [6]: Angle(3., unit=u.deg).to_string(precision=0)
Out[6]: array(u'3d00m00s', dtype=object)
```