Trial with equivalency=(u.rad,None)
Add radian_is_dimensionless function to equivalencies.py
Add test cases, and, as a result, correct is_equivalent
Also rename equivalency to angles_dimensionless
Add documentation for angles_dimensionless in units/equivalencies.rst
Correct equivalency tests
Use same test for (unit, None) equivalency in is_equivalent and _apply_equivalencies
More reliable test for (unit, None) converter