Set is_ground_content to false (#386)

mesecons_receiver/init.lua

@@ -42,6 +42,7 @@ mesecon.register_node("mesecons_receiver:receiver", {
 	drawtype = "nodebox",
 	paramtype = "light",
 	paramtype2 = "facedir",
+	is_ground_content = false,
 	sunlight_propagates = true,
 	walkable = false,
 	on_rotate = false,
@@ -89,6 +90,7 @@ mesecon.register_node("mesecons_receiver:receiver_up", {
 	drawtype = "nodebox",
 	paramtype = "light",
 	paramtype2 = "facedir",
+	is_ground_content = false,
 	sunlight_propagates = true,
 	walkable = false,
 	on_rotate = false,
@@ -132,6 +134,7 @@ mesecon.register_node("mesecons_receiver:receiver_down", {
 	drawtype = "nodebox",
 	paramtype = "light",
 	paramtype2 = "facedir",
+	is_ground_content = false,
 	sunlight_propagates = true,
 	walkable = false,
 	on_rotate = false,