Avoid invalid downcasts to SwFmtHeader instead of SwFmtFooter
Avoid invalid downcasts to SwFmtHeader instead of SwFmtFooter

...which only happened to work because the two classes have very similar layout

Change-Id: Ie4bde807a2442a95ff515b065d4f45f257a32f34

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core