add download strategy for artstation

test/unit/downloads/art_station_test.rb

@@ -0,0 +1,18 @@
+require 'test_helper'
+
+module Downloads
+  class ArtStationTest < ActiveSupport::TestCase
+    context "a download for a (small) artstation image" do
+      setup do
+        @source = "https://cdnb3.artstation.com/p/assets/images/images/003/716/071/large/aoi-ogata-hate-city.jpg?1476754974"
+        @tempfile = Tempfile.new("danbooru-test")
+        @download = Downloads::File.new(@source, @tempfile.path)
+        @download.download!
+      end
+
+      should "download the large image instead" do
+        assert_equal("https://cdnb3.artstation.com/p/assets/images/images/003/716/071/original/aoi-ogata-hate-city.jpg?1476754974", @download.source)
+      end
+    end
+  end
+end