Fix #5002: "Urls url has already been taken" when submitting duplicate urls with different capitalization

Fix URLs being normalized after checking for duplicates rather than
before, which meant that URLs that differed in capitalization weren't
detected as duplicates.

test/unit/artist_test.rb

@@ -158,6 +158,13 @@ class ArtistTest < ActiveSupport::TestCase
       assert_equal(old_url_ids, ArtistUrl.order("id").pluck(&:id))
     end
 
+    should "normalize urls before removing duplicates" do
+      @artist = create(:artist, url_string: "https://Twitter.com/o8q https://twitter.com/o8q")
+
+      assert_equal(1, @artist.urls.count)
+      assert_equal(["https://twitter.com/o8q"], @artist.urls.map(&:to_s))
+    end
+
     should "ignore pixiv.net/ and pixiv.net/img/ url matches" do
       a1 = FactoryBot.create(:artist, :name => "yomosaka", :url_string => "http://i2.pixiv.net/img18/img/evazion/14901720.png")
       a2 = FactoryBot.create(:artist, :name => "niwatazumi_bf", :url_string => "http://i2.pixiv.net/img18/img/evazion/14901720_big_p0.png")