u.deg.is_equivalent(0*u.deg) results in div-by-zero


```
u.deg.is_equivalent(0*u.deg)


---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-67-622310b0549e> in <module>()
----> 1 u.deg.is_equivalent(0*u.deg)

/Users/adam/repos/astropy/astropy/units/core.pyc in is_equivalent(self, other, equivalencies)
    727                        for u in other)
    728         else:
--> 729             other = Unit(other, parse_strict='silent')
    730 
    731         return self._is_equivalent(other, equivalencies)

/Users/adam/repos/astropy/astropy/units/core.pyc in __call__(self, s, represents, format, namespace, doc, parse_strict)
   1683                 s = CompositeUnit(s.value * s.unit.scale,
   1684                                   bases=s.unit.bases,
-> 1685                                   powers=s.unit.powers)
   1686 
   1687         # now decide what we really need to do; define derived Unit?

/Users/adam/repos/astropy/astropy/units/core.pyc in __init__(self, scale, bases, powers, decompose, decompose_bases, _error_check)
   1889         # off.
   1890         if _error_check:
-> 1891             scale = sanitize_scale(scale)
   1892             for base in bases:
   1893                 if not isinstance(base, UnitBase):

/Users/adam/repos/astropy/astropy/units/utils.pyc in sanitize_scale(scale)
    143                 scale = scale.real
    144         else:
--> 145             if is_effectively_unity(scale.real/scale.imag + 1):
    146                 scale = complex(0., scale.imag)
    147 

ZeroDivisionError: float division by zero
```