Rubocop fix: Perfomance/UnfreezeString (#26217)

app/lib/rss/builder.rb

@@ -14,13 +14,14 @@ class RSS::Builder
   end
 
   def to_xml
-    ('<?xml version="1.0" encoding="UTF-8"?>'.dup << Ox.dump(wrap_in_document, effort: :tolerant)).force_encoding('UTF-8')
+    Ox.dump(wrap_in_document, effort: :tolerant).force_encoding('UTF-8')
   end
 
   private
 
   def wrap_in_document
     Ox::Document.new(version: '1.0').tap do |document|
+      document << xml_instruct
       document << Ox::Element.new('rss').tap do |rss|
         rss['version']        = '2.0'
         rss['xmlns:webfeeds'] = 'http://webfeeds.org/rss/1.0'
@@ -30,4 +31,11 @@ class RSS::Builder
       end
     end
   end
+
+  def xml_instruct
+    Ox::Instruct.new(:xml).tap do |instruct|
+      instruct[:version] = '1.0'
+      instruct[:encoding] = 'UTF-8'
+    end
+  end
 end

app/lib/text_formatter.rb

@@ -75,7 +75,7 @@ class TextFormatter
       entity[:indices].first
     end
 
-    result = ''.dup
+    result = +''
 
     last_index = entities.reduce(0) do |index, entity|
       indices = entity[:indices]