moebooru: fix preview_urls to fallback to image_urls.

Fix the moebooru strategy to fallback to returning the image url if we
can't find the preview url. Fixes iqdb lookups failing in some cases
because the strategy didn't return a valid url for preview_url.

app/logical/sources/strategies/moebooru.rb

@@ -62,7 +62,7 @@ module Sources
       end
 
       def preview_urls
-        return [] unless post_md5.present?
+        return image_urls unless post_md5.present?
         ["https://#{file_host}/data/preview/#{post_md5[0..1]}/#{post_md5[2..3]}/#{post_md5}.jpg"]
       end
 

test/unit/sources/moebooru_test.rb

@@ -28,6 +28,15 @@ module Sources
         end
       end
 
+      context "A 'https://assets.yande.re/preview/:hh/:hh/:file.jpg' preview url" do
+        should "return a non-empty list of preview_urls" do
+          url = "https://assets.yande.re/data/preview/7c/d1/7cd124fc28203233cce3bade26651d43.jpg"
+          site = Sources::Strategies.find(url)
+
+          assert_equal([url], site.preview_urls)
+        end
+      end
+
       context "Fetching data for an active yande.re .jpg post" do
         should "work" do
           @prev = "https://files.yande.re/data/preview/7e/cf/7ecfdead705d7b956b26b1d37b98d089.jpg"