push header merge down to a private method so that live responses can have their own header object

actionpack/lib/action_controller/metal/live.rb

@@ -81,11 +81,6 @@ def to_hash
         end
       end
 
-      def initialize(status = 200, header = {}, body = [])
-        header = Header.new self, header
-        super(status, header, body)
-      end
-
       def commit!
         headers.freeze
         super
@@ -98,6 +93,10 @@ def build_buffer(response, body)
         body.each { |part| buf.write part }
         buf
       end
+
+      def merge_default_headers(original, default)
+        Header.new self, super
+      end
     end
 
     def process(name)

actionpack/lib/action_dispatch/http/response.rb

@@ -97,9 +97,7 @@ def closed?
     def initialize(status = 200, header = {}, body = [])
       super()
 
-      if self.class.default_headers.respond_to?(:merge)
-        header = self.class.default_headers.merge(header)
-      end
+      header = merge_default_headers(header, self.class.default_headers)
 
       self.body, self.header, self.status = body, header, status
 
@@ -243,6 +241,12 @@ def cookies
 
   private
 
+    def merge_default_headers(original, default)
+      return original unless default.respond_to?(:merge)
+
+      default.merge(original)
+    end
+
     def build_buffer(response, body)
       Buffer.new response, body
     end

actionpack/test/dispatch/live_response_test.rb

@@ -14,6 +14,17 @@ def test_header_merge
         refute_equal header, @response.header
       end
 
+      def test_initialize_with_default_headers
+        r = Class.new(Live::Response) do
+          def self.default_headers
+            { 'omg' => 'g' }
+          end
+        end
+
+        header = r.new.header
+        assert_kind_of(ActionController::Live::Response::Header, header)
+      end
+
       def test_parallel
         latch = ActiveSupport::Concurrency::Latch.new