Write test to ensure that we export sheet tab color settings correctly.
Write test to ensure that we export sheet tab color settings correctly.

Previously, we would incorrectly export a sheet tab without any color set
as a tab with white color.

Change-Id: I97e59a0d8e2fba6db64149f53acfe4331a5ce2ba

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