fix a bug in column_spec, fix kable_as_image in windows
diff --git a/R/column_spec.R b/R/column_spec.R
index 615a22e..97dcd06 100644
--- a/R/column_spec.R
+++ b/R/column_spec.R
@@ -183,7 +183,7 @@
     color <- sprintf("\\\\color{%s}", color)
   }
 
-  if (!is.null(color)) {
+  if (!is.null(background)) {
     background <- sprintf("\\\\columncolor{%s}", background)
   }