add two more tests to make sure that conversion from air wavelength -> wavelength results in an air-to-vac conversion
add two more tests to make sure that conversion from air wavelength ->
wavelength results in an air-to-vac conversion

Conflicts:
	spectral_cube/tests/test_spectral_cube.py

e-koch/spectral-cube,jzuhone/spectral-cube,keflavich/spectral-cube,radio-astro-tools/spectral-cube,low-sky/spectral-cube