remove html_types set

Now that `all` has it's own object, we don't need the html_types Set.

actionpack/lib/action_dispatch/http/mime_type.rb

-require 'set'
 require 'singleton'
 require 'active_support/core_ext/module/attribute_accessors'
 require 'active_support/core_ext/string/starts_ends_with'
@@ -95,9 +94,6 @@ def const_defined?(sym, inherit = true)
   #     end
   #   end
   class Type
-    @@html_types = Set.new [:html, :all]
-    cattr_reader :html_types
-
     attr_reader :symbol
 
     @register_callbacks = []
@@ -301,7 +297,7 @@ def to_sym
     end
 
     def ref
-      to_sym || to_s
+      symbol || to_s
     end
 
     def ===(list)
@@ -328,7 +324,7 @@ def =~(mime_type)
     end
 
     def html?
-      @@html_types.include?(to_sym) || @string =~ /html/
+      symbol == :html || @string =~ /html/
     end
 
     def all?; false; end

actionpack/test/dispatch/mime_type_test.rb

@@ -165,7 +165,7 @@ class MimeTypeTest < ActiveSupport::TestCase
       assert mime.respond_to?("#{type}?"), "#{mime.inspect} does not respond to #{type}?"
       assert_equal type, mime.symbol, "#{mime.inspect} is not #{type}?"
       invalid_types = types - [type]
-      invalid_types.delete(:html) if Mime::Type.html_types.include?(type)
+      invalid_types.delete(:html)
       invalid_types.each { |other_type|
         assert_not_equal mime.symbol, other_type, "#{mime.inspect} is #{other_type}?"
       }