added tests for optional arguments
added tests for optional arguments

larrybradley/astropy,aleksandr-bakanov/astropy,pllim/astropy,StuartLittlefair/astropy,bsipocz/astropy,lpsinger/astropy,dhomeier/astropy,bsipocz/astropy,saimn/astropy,pllim/astropy,dhomeier/astropy,dhomeier/astropy,MSeifert04/astropy,stargaser/astropy,lpsinger/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,stargaser/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,saimn/astropy,lpsinger/astropy,astropy/astropy,dhomeier/astropy,StuartLittlefair/astropy,astropy/astropy,lpsinger/astropy,saimn/astropy,mhvk/astropy,stargaser/astropy,StuartLittlefair/astropy,MSeifert04/astropy,aleksandr-bakanov/astropy,bsipocz/astropy,StuartLittlefair/astropy,pllim/astropy,pllim/astropy,astropy/astropy,mhvk/astropy,mhvk/astropy,stargaser/astropy,MSeifert04/astropy,pllim/astropy,bsipocz/astropy,mhvk/astropy,larrybradley/astropy,StuartLittlefair/astropy,larrybradley/astropy,astropy/astropy,saimn/astropy,MSeifert04/astropy,astropy/astropy,dhomeier/astropy,larrybradley/astropy,saimn/astropy,mhvk/astropy