tag aliases: fix bug in alias resolution.

Bug: Searching for an aliased tag returns an empty page instead of
showing the results for the real tag.

Cause: The query parsing code runs the search string through
`.mb_chars.downcase` before calling `TagAlias.to_aliased`, so the input
to `to_aliased` is actually a ActiveSupport::Multibyte::Chars object.
This breaks the `aliases[name]` hash lookup because `name` is not a
plain string.

Fixup for c7bcce429.

app/models/tag_alias.rb

@@ -44,8 +44,8 @@ class TagAlias < TagRelationship
   end
 
   def self.to_aliased(names)
-    names = Array(names)
-    return names if names.empty?
+    names = Array(names).map(&:to_s)
+    return [] if names.empty?
     aliases = active.where(antecedent_name: names).map { |ta| [ta.antecedent_name, ta.consequent_name] }.to_h
     names.map { |name| aliases[name] || name }
   end