Fix default headers in test responses

Fixes regression in #18423. Merge default headers for new responses,
but don't merge when creating a response from the last session request.

hat tip @senny :heart:

1 files changed, 3 insertions, 5 deletions

diff --git a/actionpack/lib/action_dispatch/http/response.rb b/actionpack/lib/action_dispatch/http/response.rb
index 4061ea71a3..a895d1ab18 100644
--- a/actionpack/lib/action_dispatch/http/response.rb
+++ b/actionpack/lib/action_dispatch/http/response.rb
@@ -113,10 +113,10 @@ module ActionDispatch # :nodoc:
     # The underlying body, as a streamable object.
     attr_reader :stream
 
-    def initialize(status = 200, header = {}, body = [])
+    def initialize(status = 200, header = {}, body = [], default_headers: self.class.default_headers)
       super()
 
-      header = merge_default_headers(header, self.class.default_headers)
+      header = merge_default_headers(header, default_headers)
 
       self.body, self.header, self.status = body, header, status
 
@@ -308,9 +308,7 @@ module ActionDispatch # :nodoc:
     end
 
     def merge_default_headers(original, default)
-      return original unless default.respond_to?(:merge)
-
-      default.merge(original)
+      default.respond_to?(:merge) ? default.merge(original) : original
     end
 
     def build_buffer(response, body)