Fix #278 - Use mb_chars.downcase on hashtags

app/services/process_hashtags_service.rb

@@ -4,7 +4,7 @@ class ProcessHashtagsService < BaseService
   def call(status, tags = [])
     tags = status.text.scan(Tag::HASHTAG_RE).map(&:first) if status.local?
 
-    tags.map(&:downcase).uniq.each do |tag|
+    tags.map { |str| str.mb_chars.downcase }.uniq.each do |tag|
       status.tags << Tag.where(name: tag).first_or_initialize(name: tag)
     end
   end