Mime::Mimes#symbols should return a always up to date reference:

- Original issue was reported in https://github.com/rails/rails/issues/38094
  and a fix attempted in https://github.com/rails/rails/pull/38126 but
  it's not the proper fix I think.

  TL;DR Is that `ActionView::Base.default_formats` holds a copy of
  mime symbols at the time ActionView::Base is loaded.
  So if you try to register mime types after ActionView Base is loaded
  then it won't work.

  ```ruby
    ActionView::Base.default_formats ||= Mime::SET.symbols # Note that this is automatically done when ActionView get loaded 22483b86a6/actionpack/lib/action_dispatch.rb (L117)

   Mime::Type.register_alias "application/xhtml+xml", :foobar
   puts ActionView::base.defaults_formats.include?(:foobar) # => false
  ```

  Same issue if you try to unregister a mime after ActionView is loaded.
  That's what was happening in the flaky test:

  ```
   Mime::Type.register_alias "application/xhtml+xml", :foobar
   ActionView::Base.default_formats ||= Mime::SET.symbols

   puts ActionView::base.defaults_formats.include?(:foobar) # => true

   Mime::Type.unregister(:foobar)
   puts ActionView::base.defaults_formats.include?(:foobar) # => true
  ```

  ### Solution

  Return a refence to `@symbols` which is updated each time a new mime is
  registered/unregistered.

actionpack/lib/action_dispatch/http/mime_type.rb

@@ -5,11 +5,13 @@ require "active_support/core_ext/string/starts_ends_with"
 
 module Mime
   class Mimes
+    attr_reader :symbols
+
     include Enumerable
 
     def initialize
       @mimes = []
-      @symbols = nil
+      @symbols = []
     end
 
     def each
@@ -18,15 +20,16 @@ module Mime
 
     def <<(type)
       @mimes << type
-      @symbols = nil
+      @symbols << type.to_sym
     end
 
     def delete_if
-      @mimes.delete_if { |x| yield x }.tap { @symbols = nil }
-    end
-
-    def symbols
-      @symbols ||= map(&:to_sym)
+      @mimes.delete_if do |x|
+        if yield x
+          @symbols.delete(x.to_sym)
+          true
+        end
+      end
     end
   end
 

actionpack/test/controller/mime/accept_format_test.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require "abstract_unit"
-ActionView::Base.default_formats ||= Mime::SET.symbols
 
 class StarStarMimeController < ActionController::Base
   layout nil

actionpack/test/dispatch/mime_type_test.rb

@@ -227,4 +227,14 @@ class MimeTypeTest < ActiveSupport::TestCase
       Mime::Type.new(nil)
     end
   end
+
+  test "holds a reference to mime symbols" do
+    old_symbols = Mime::SET.symbols
+    Mime::Type.register_alias "application/xhtml+xml", :foobar
+    new_symbols = Mime::SET.symbols
+
+    assert_same(old_symbols, new_symbols)
+  ensure
+    Mime::Type.unregister(:foobar)
+  end
 end