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.

app/logical/sources/strategies/tumblr.rb

@@ -105,12 +105,15 @@ module Sources::Strategies
 
     def tags
       post[:tags].to_a.map do |tag|
-        # normalize tags: space, underscore, and hyphen are equivalent in tumblr tags.
-        etag = tag.gsub(/[ _-]/, "_")
-        [etag, "https://tumblr.com/tagged/#{CGI.escape(etag)}"]
+        [tag, "https://tumblr.com/tagged/#{CGI.escape(tag)}"]
       end.uniq
     end
 
+    def normalize_tag(tag)
+      tag = tag.tr("-", "_")
+      super(tag)
+    end
+
     def dtext_artist_commentary_desc
       DText.from_html(artist_commentary_desc).strip
     end