sources: don't escape Unicode characters in tag search URLs.

Fix it so that Unicode characters aren't unnecessarily percent-encoded when generating tag search
URLs. For example, generate URLs like this:

* https://www.pixiv.net/tags/オリジナル/artworks

Not like this:

* https://www.pixiv.net/tags/%E3%82%AA%E3%83%AA%E3%82%B8%E3%83%8A%E3%83%AB/artworks

test/unit/sources/pixiv_test.rb

@@ -94,10 +94,8 @@ module Sources
 
         should "get the tags" do
           pixiv_tags  = @site.tags.map(&:first)
-          pixiv_links = @site.tags.map(&:last)
 
           assert_equal(%w[漫画 test], pixiv_tags)
-          assert_contains(pixiv_links, /search\.php/)
         end
 
         should "get the artist commentary" do