live response headers can be merged with a hash

actionpack/lib/action_controller/metal/live.rb

@@ -72,6 +72,10 @@ def []=(k,v)
           super
         end
 
+        def merge(other)
+          self.class.new @response, __getobj__.merge(other)
+        end
+
         def to_hash
           __getobj__.dup
         end

actionpack/test/dispatch/live_response_test.rb

@@ -8,6 +8,12 @@ def setup
         @response = Live::Response.new
       end
 
+      def test_header_merge
+        header = @response.header.merge('Foo' => 'Bar')
+        assert_kind_of(ActionController::Live::Response::Header, header)
+        refute_equal header, @response.header
+      end
+
       def test_parallel
         latch = ActiveSupport::Concurrency::Latch.new