prefer composition over inheritence

actionpack/lib/action_dispatch/http/headers.rb

 module ActionDispatch
   module Http
-    class Headers < ::Hash
+    class Headers
+      include Enumerable
+
       @@env_cache = Hash.new { |h,k| h[k] = "HTTP_#{k.upcase.gsub(/-/, '_')}" }
 
       def initialize(*args)
-
-        if args.size == 1 && args[0].is_a?(Hash)
-          super()
-          update(args[0])
-        else
-          super
-        end
+        @headers = args.first || {}
       end
 
       def [](header_name)
-        super env_name(header_name)
+        @headers[env_name(header_name)]
       end
 
+      def []=(k,v); @headers[k] = v; end
+      def key?(k); @headers.key? k; end
+      alias :include? :key?
+
       def fetch(header_name, default=nil, &block)
-        super env_name(header_name), default, &block
+        @headers.fetch env_name(header_name), default, &block
+      end
+
+      def each(&block)
+        @headers.each(&block)
       end
 
       private
         # Converts a HTTP header name to an environment variable name if it is
         # not contained within the headers hash.
         def env_name(header_name)
-          include?(header_name) ? header_name : @@env_cache[header_name]
+          @headers.include?(header_name) ? header_name : @@env_cache[header_name]
         end
     end
   end

actionpack/test/dispatch/header_test.rb

@@ -7,6 +7,22 @@ def setup
     )
   end
 
+  def test_each
+    headers = []
+    @headers.each { |pair| headers << pair }
+    assert_equal [["HTTP_CONTENT_TYPE", "text/plain"]], headers
+  end
+
+  def test_setter
+    @headers['foo'] = "bar"
+    assert_equal "bar", @headers['foo']
+  end
+
+  def test_key?
+    assert @headers.key?('HTTP_CONTENT_TYPE')
+    assert @headers.include?('HTTP_CONTENT_TYPE')
+  end
+
   test "content type" do
     assert_equal "text/plain", @headers["Content-Type"]
     assert_equal "text/plain", @headers["content-type"]