Introduce `Headers#add`. Move `Response#add_header` upstream.

* Introduce `ActionDispatch::Http::Headers#add` to add a value to
  a multivalued header.
* Move `Response#add_header` upstream: https://github.com/rack/rack/pull/957
* Match upstream `Response#have_header?` -> `#has_header?` name change.

1 files changed, 5 insertions, 0 deletions

diff --git a/actionpack/lib/action_dispatch/http/headers.rb b/actionpack/lib/action_dispatch/http/headers.rb
index 9a3aaca3f0..12f81dc1a5 100644
--- a/actionpack/lib/action_dispatch/http/headers.rb
+++ b/actionpack/lib/action_dispatch/http/headers.rb
@@ -49,6 +49,11 @@ module ActionDispatch
         @req.set_header env_name(key), value
       end
 
+      # Add a value to a multivalued header like Vary or Accept-Encoding.
+      def add(key, value)
+        @req.add_header env_name(key), value
+      end
+
       def key?(key)
         @req.has_header? env_name(key)
       end