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
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