Make steradian and radian ** 2 units equivalent?

I wanted to compute the area of the whole sky in `deg^2` with `astropy.units`.
Currently this gives a `UnitsException` because steradian `sr` and `radian^2` are not equivalent.
Is there a reason they shouldn't be equivalent?
If not, could you please make them equivalent?

```
In [1]: import astropy.units as u

In [2]: u.sr.is_equivalent(u.radian ** 2)
Out[2]: False

In [3]: import numpy as np

In [4]: all_sky_solid_angle = (4 * np.pi * u.sr).to(u.deg ** 2)
ERROR: UnitsException: 'sr' (solid angle) and 'deg2' are not convertible [astropy.units.core]
---------------------------------------------------------------------------
UnitsException                            Traceback (most recent call last)
<ipython-input-4-a177453d6e1d> in <module>()
----> 1 all_sky_solid_angle = (4 * np.pi * u.sr).to(u.deg ** 2)

/Users/deil/Library/Python/2.7/lib/python/site-packages/astropy/units/quantity.pyc in to(self, unit)
     81             object or a string parseable by the `units` package.
     82         """
---> 83         new_val = self.unit.to(unit, self.value)
     84         new_unit = Unit(unit)
     85         return Quantity(new_val, new_unit)

/Users/deil/Library/Python/2.7/lib/python/site-packages/astropy/units/core.pyc in to(self, other, value, equivs)
    364         """
    365         other = Unit(other)
--> 366         return self.get_converter(other, equivs=equivs)(value)
    367 
    368     def in_units(self, other, value=1.0, equivs=[]):

/Users/deil/Library/Python/2.7/lib/python/site-packages/astropy/units/core.pyc in get_converter(self, other, equivs)
    332         except UnitsException:
    333             return self._apply_equivalences(
--> 334                 self, other, equivs)
    335         return lambda val: scale * _condition_arg(val)
    336 

/Users/deil/Library/Python/2.7/lib/python/site-packages/astropy/units/core.pyc in _apply_equivalences(self, unit, other, equivs)
    298         raise UnitsException(
    299             "{0} and {1} are not convertible".format(
--> 300                 unit_str, other_str))
    301 
    302     def get_converter(self, other, equivs=[]):

UnitsException: 'sr' (solid angle) and 'deg2' are not convertible
```