Don't make rexml an dependency of activesupport

rexml is only used in the xml_mini backend and it should be the users
choice if they want to use that feature or not. If they do we will warn
them that installing rexml is needed like we do with all backends.

Gemfile

@@ -48,6 +48,7 @@ gem "dalli"
 gem "listen", "~> 3.2", require: false, github: "guard/listen"
 gem "libxml-ruby", platforms: :ruby
 gem "connection_pool", require: false
+gem "rexml", require: false
 
 # for railties app_generator_test
 gem "bootsnap", ">= 1.4.4", require: false

Gemfile.lock

@@ -95,7 +95,6 @@ PATH
       concurrent-ruby (~> 1.0, >= 1.0.2)
       i18n (>= 1.6, < 2)
       minitest (>= 5.1)
-      rexml
       tzinfo (~> 2.0)
       zeitwerk (~> 2.3)
     rails (6.1.0.alpha)
@@ -607,6 +606,7 @@ DEPENDENCIES
   redis-namespace!
   resque
   resque-scheduler
+  rexml
   rouge
   rubocop (>= 0.90)
   rubocop-packaging

activesupport/activesupport.gemspec

@@ -38,5 +38,4 @@ Gem::Specification.new do |s|
   s.add_dependency "concurrent-ruby", "~> 1.0", ">= 1.0.2"
   s.add_dependency "zeitwerk",        "~> 2.3"
   s.add_dependency "minitest",        ">= 5.1"
-  s.add_dependency "rexml"
 end

activesupport/lib/active_support/xml_mini/rexml.rb

@@ -25,7 +25,7 @@ module ActiveSupport
       if data.eof?
         {}
       else
-        silence_warnings { require "rexml/document" } unless defined?(REXML::Document)
+        require_rexml unless defined?(REXML::Document)
         doc = REXML::Document.new(data)
 
         if doc.root
@@ -38,6 +38,14 @@ module ActiveSupport
     end
 
     private
+
+      def require_rexml
+        silence_warnings { require "rexml/document" }
+      rescue LoadError => e
+        $stderr.puts "You don't have rexml installed in your application. Please add it to your Gemfile and run bundle install"
+        raise e
+      end
+
       # Convert an XML element and merge into the hash
       #
       # hash::