Let's not export sheet tab color when it's not set.
Let's not export sheet tab color when it's not set.

COL_AUTO is the default value which is treated as color not being set.  The
old code would export it as a white color which is not correct.

Change-Id: Ib68fccfb228333f2059edc4cfffeed90880ebc60

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