gelbooru: fix exception in md5-based post urls.

Fix exception when trying to get the image URL for sources like
https://gelbooru.com/index.php?page=post&s=list&md5=04f2767c64593c3030ce74ecc2528704.

app/logical/source/extractor/gelbooru.rb

@@ -59,7 +59,7 @@ module Source
       end
 
       memoize def post_id_from_md5
-        return nil unless parsed_url.image_url? && parsed_url.page_url.present?
+        return nil unless parsed_url.page_url.present?
 
         response = http.cache(1.minute).head(parsed_url.page_url)
         return nil unless response.status == 200

test/unit/sources/gelbooru_test.rb

@@ -54,5 +54,18 @@ module Sources
         download_size: 480_621,
       )
     end
+
+    context "A Gelbooru md5 page url" do
+      strategy_should_work(
+        "https://gelbooru.com/index.php?page=post&s=list&md5=04f2767c64593c3030ce74ecc2528704",
+        image_urls: ["https://img3.gelbooru.com/images/04/f2/04f2767c64593c3030ce74ecc2528704.jpg"],
+        artist_name: "灰色灰烬bot",
+        profile_url: "https://www.pixiv.net/users/3330425",
+        tags: %w[1girl back_bow bangs black_pantyhose blue_bow blue_hair blue_ribbon boots bow cape chibi chinese_commentary closed_eyes full_body hair_between_eyes hair_ribbon hat hatsune_miku indai_(3330425) on_ground pantyhose pom_pom_(clothes) rabbit rabbit_yukine rating:general ribbon simple_background sitting solo twintails vocaloid white_background white_cape white_headwear witch_hat yuki_miku yuki_miku_(2014) 初音ミク 雪ミク],
+        artist_commentary_title: "2010~2021雪ミク",
+        artist_commentary_desc: "动作参考@速写班长",
+        download_size: 480_621,
+      )
+    end
   end
 end