Fix #4016: Translated tags failing to find some tags.

* Normalize spaces to underscores when saving other names. Preserve case
  since case can be significant.

* Fix WikiPage#other_names_include to search case-insensitively (note:
  this prevents using the index).

* Fix sources to return the raw tags in `#tags` and the normalized tags
  in `#normalized_tags`. The normalized tags are the tags that will be
  matched against other names.

test/unit/sources/tumblr_test.rb

@@ -20,8 +20,9 @@ module Sources
       end
 
       should "get the tags" do
-        tags = [["tag", "https://tumblr.com/tagged/tag"], ["red_hair", "https://tumblr.com/tagged/red_hair"]]
-        assert_equal(tags, @site.tags)
+        tags = ["tag", "red hair", "red-hair", "red_hair"]
+        assert_equal(tags, @site.tags.map(&:first))
+        assert_equal(["red_hair", "tag"], @site.normalized_tags)
       end
 
       should "get the commentary" do
@@ -100,8 +101,9 @@ module Sources
       end
 
       should "get the tags" do
-        tags = [["tag", "https://tumblr.com/tagged/tag"], ["red_hair", "https://tumblr.com/tagged/red_hair"]]
-        assert_equal(tags, @site.tags)
+        tags = ["tag", "red hair", "red-hair", "red_hair"]
+        assert_equal(tags, @site.tags.map(&:first))
+        assert_equal(["red_hair", "tag"], @site.normalized_tags)
       end
     end