Test ignore_blank_cards in HeaderDiff, HDUDiff, FITSDiff
Test ignore_blank_cards in HeaderDiff, HDUDiff, FITSDiff

Signed-off-by: Conor MacBride <92053a0ea29d3a95c7f6d0021e7464a2159ae789@macbride.me>

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