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
Change build-in equivalency test to one that uses a class
This since objects can no longer be initialised with equivalencies
Change name to dimensionless_angles
Remove xfail equivalency context manager test given renaming in #1662
Merge remote-tracking branch 'mhvk/units-radian-is-unity-equivalency'

Conflicts:
	CHANGES.rst