Pass fill value to legend key icons (#3781)

* Fill legend key icons. Closes #3455.

Author: 808sAndBR

R/legend-draw.r

@@ -137,6 +137,8 @@ draw_key_path <- function(data, params, size) {
   segmentsGrob(0.1, 0.5, 0.9, 0.5,
     gp = gpar(
       col = alpha(data$colour %||% data$fill %||% "black", data$alpha),
+      fill = alpha(params$arrow.fill %||% data$colour
+                   %||% data$fill %||% "black", data$alpha),
       lwd = (data$size %||% 0.5) * .pt,
       lty = data$linetype %||% 1,
       lineend = "butt"