FormatBackground was always returning the actual color
instead of AUTO when the color was COL_AUTO because ConvertColor
is being passed Color.GetRGBColor() instead of just Color.

There are three sections here:
-XML_fillcolor: looks like it accepts values like "green" and
"teal". Unclear from documenation whether auto is allowed.
-XML_srgbClr: only accepts rgb number, not auto
-XML_fill: needs this fix - test document available.

Since the section can't globally handle "auto", I left XML_fillcolor
alone, waiting for a proof bug report.

Change-Id: Idecb6a32fba814a6fb3aeb6cc015d793ba6f8cf9
Reviewed-on: https://gerrit.libreoffice.org/43975Reviewed-by: Justin Luth <justin_luth@sil.org>
Tested-by: Justin Luth <justin_luth@sil.org>