Only update MaskedColumn fill_value when input fill_value is not default
Only update MaskedColumn fill_value when input fill_value is not default

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