Defer cleaning _header until we are actively storing a new EXTNAME
Defer cleaning _header until we are actively storing a new EXTNAME

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