pixiv: fix fetch source data failing on fanbox images.

ref: https://danbooru.donmai.us/forum_posts/150000

app/logical/pixiv_api_client.rb

@@ -128,7 +128,7 @@ class PixivApiClient
     end
 
     def moniker
-      raise NotImplementedError
+      ""
     end
 
     def page_count

test/unit/sources/pixiv_test.rb

@@ -74,6 +74,17 @@ module Sources
         end
       end
 
+      context "A https://www.pixiv.net/fanbox/creator/*/post/* source" do
+        should "work" do
+          @site = Sources::Strategies.find("http://www.pixiv.net/fanbox/creator/554149/post/82555")
+
+          assert_equal("TYONE(お仕事募集中)", @site.artist_name)
+          assert_equal("https://www.pixiv.net/member.php?id=554149", @site.profile_url)
+          assert_equal("https://fanbox.pixiv.net/images/post/82555/Lyyeb6dDLcQZmy09nqLZapuS.jpeg", @site.image_url)
+          assert_nothing_raised { @site.to_h }
+        end
+      end
+
       context "fetching source data for a new manga image" do
         setup do
           get_source("https://www.pixiv.net/member_illust.php?mode=medium&illust_id=65981735")