activitypub: case transform: support _misskey keys without messing them up

Author: kaniini

app/lib/activitypub/case_transform.rb

@@ -14,6 +14,8 @@ def camel_lower(value)
       when String
         camel_lower_cache[value] ||= if value.start_with?('_:')
                                        '_:' + value.gsub(/\A_:/, '').underscore.camelize(:lower)
+                                     elsif value.start_with?('_')
+                                       value
                                      else
                                        value.underscore.camelize(:lower)
                                      end