fixed bug in constant model tag
fixed bug in constant model tag

mhvk/astropy,saimn/astropy,pllim/astropy,pllim/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,dhomeier/astropy,StuartLittlefair/astropy,larrybradley/astropy,astropy/astropy,mhvk/astropy,lpsinger/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,pllim/astropy,saimn/astropy,larrybradley/astropy,dhomeier/astropy,mhvk/astropy,larrybradley/astropy,dhomeier/astropy,mhvk/astropy,astropy/astropy,pllim/astropy,StuartLittlefair/astropy,astropy/astropy,dhomeier/astropy,astropy/astropy,saimn/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,StuartLittlefair/astropy,astropy/astropy,dhomeier/astropy,StuartLittlefair/astropy,saimn/astropy,lpsinger/astropy,mhvk/astropy,larrybradley/astropy,saimn/astropy,pllim/astropy