Add test for conversion to own unit.
Add test for conversion to own unit.

StuartLittlefair/astropy,saimn/astropy,StuartLittlefair/astropy,saimn/astropy,astropy/astropy,astropy/astropy,mhvk/astropy,astropy/astropy,StuartLittlefair/astropy,lpsinger/astropy,larrybradley/astropy,pllim/astropy,saimn/astropy,mhvk/astropy,lpsinger/astropy,mhvk/astropy,mhvk/astropy,astropy/astropy,saimn/astropy,lpsinger/astropy,pllim/astropy,larrybradley/astropy,pllim/astropy,larrybradley/astropy,lpsinger/astropy,pllim/astropy,StuartLittlefair/astropy,pllim/astropy,lpsinger/astropy,larrybradley/astropy,astropy/astropy,saimn/astropy,mhvk/astropy,larrybradley/astropy,StuartLittlefair/astropy