change inheritance to composition

Changes `Mimes` to compose a set rather than inherit from array.  With
this change we don't need to define as many methods, so ISEQ memory is
saved.  Also it is clear which methods break the set cache.

actionpack/lib/action_dispatch/http/mime_type.rb

@@ -5,20 +5,29 @@
 require 'active_support/deprecation'
 
 module Mime
-  class Mimes < Array
-    def symbols
-      @symbols ||= map(&:to_sym)
+  class Mimes
+    include Enumerable
+
+    def initialize
+      @mimes = []
+      @symbols = nil
     end
 
-    %w(<< concat shift unshift push pop []= clear compact! collect!
-    delete delete_at delete_if flatten! map! insert reject! reverse!
-    replace slice! sort! uniq!).each do |method|
-      module_eval <<-CODE, __FILE__, __LINE__ + 1
-        def #{method}(*)
-          @symbols = nil
-          super
-        end
-      CODE
+    def each
+      @mimes.each { |x| yield x }
+    end
+
+    def <<(type)
+      @mimes << type
+      @symbols = nil
+    end
+
+    def delete_if
+      @mimes.delete_if { |x| yield x }.tap { @symbols = nil }
+    end
+
+    def symbols
+      @symbols ||= map(&:to_sym)
     end
   end
 
@@ -214,12 +223,13 @@ def register(string, symbol, mime_type_synonyms = [], extension_synonyms = [], s
 
         SET << new_mime
 
-        ([string] + mime_type_synonyms).each { |str| LOOKUP[str] = SET.last } unless skip_lookup
-        ([symbol] + extension_synonyms).each { |ext| EXTENSION_LOOKUP[ext.to_s] = SET.last }
+        ([string] + mime_type_synonyms).each { |str| LOOKUP[str] = new_mime } unless skip_lookup
+        ([symbol] + extension_synonyms).each { |ext| EXTENSION_LOOKUP[ext.to_s] = new_mime }
 
         @register_callbacks.each do |callback|
           callback.call(new_mime)
         end
+        new_mime
       end
 
       def parse(accept_header)

actionpack/test/dispatch/mime_type_test.rb

@@ -95,8 +95,9 @@ class MimeTypeTest < ActiveSupport::TestCase
 
   test "custom type" do
     begin
-      Mime::Type.register("image/foo", :foo)
-      assert_equal Mime::Type[:FOO], Mime::SET.last
+      type = Mime::Type.register("image/foo", :foo)
+      assert_equal Mime::Type[:FOO], type
+      assert Mime::Type.registered?(:FOO)
     ensure
       Mime::Type.unregister(:FOO)
     end