sources: fix sources sometimes choosing wrong strategy (fix #3968)

Fix sources choosing the wrong strategy when the referer belongs to a
different site (for example, when uploading a twitter post with a pixiv
referer).

* Fix `match?` to only consider the main url, not the referer.

* Change `match?` to match against a list of domains given by the `domains` method.

* Change `match?` to an instance method.

test/unit/sources/twitter_test.rb

@@ -229,5 +229,14 @@ module Sources
         assert_equal(desc2, site.dtext_artist_commentary_desc)
       end
     end
+
+    context "A twitter post with a pixiv referer" do
+      should "use the twitter strategy" do
+        site = Sources::Strategies.find("https://twitter.com/Mityubi/status/849630665603665920", "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=56735489")
+
+        assert_equal(site.site_name, "Twitter")
+        assert_equal("https://pbs.twimg.com/media/C8p-gPhVoAMZupS.png:orig", site.image_url)
+      end
+    end
   end
 end