Add `Vary: Accept` header when rendering

Problem description (quoted from @rafaelfranca's excellent explanation in https://github.com/rails/jquery-ujs/issues/318#issuecomment-88129005):

> Let say that we requested /tasks/1 using Ajax, and the previous page has the same url. When we click the back button the browser tries to get the response from its cache and it gets the javascript response. With vary we "fix" this behavior because we are telling the browser that the url is the same but it is not from the same type what will skip the cache.

And there's a Rails issue discussing about this problem as well https://github.com/rails/rails/issues/25842

Also, according to [RFC 7231 7.1.4](https://tools.ietf.org/html/rfc7231#section-7.1.4)

>  An origin server SHOULD send a Vary header field when its algorithm
>  for selecting a representation varies based on aspects of the request
>  message other than the method and request target

we should add `Vary: Accept` header when determining content based on the `Accept` header.

Although adding such header by default could cause unnecessary cache invalidation. But this PR only adds the header if:
- The format param is not provided
- The request is a `xhr` request
- The request has accept headers and the headers are valid

So if the user
- sends request with explicit format, like `/users/1.json`
- or sends a normal request (non xhr)
- or doesn't specify accept headers

then the header won't be added.

See the discussion in https://github.com/rails/rails/issues/25842 and
https://github.com/rails/rails/pull/36213 for more details.

1 files changed, 11 insertions, 7 deletions

diff --git a/actionpack/lib/action_dispatch/http/mime_negotiation.rb b/actionpack/lib/action_dispatch/http/mime_negotiation.rb
index a2cac49082..ac0ff133eb 100644
--- a/actionpack/lib/action_dispatch/http/mime_negotiation.rb
+++ b/actionpack/lib/action_dispatch/http/mime_negotiation.rb
@@ -62,13 +62,7 @@ module ActionDispatch
 
       def formats
         fetch_header("action_dispatch.request.formats") do |k|
-          params_readable = begin
-                              parameters[:format]
-                            rescue *RESCUABLE_MIME_FORMAT_ERRORS
-                              false
-                            end
-
-          v = if params_readable
+          v = if params_readable?
             Array(Mime[parameters[:format]])
           elsif use_accept_header && valid_accept_header
             accepts
@@ -153,9 +147,19 @@ module ActionDispatch
         order.include?(Mime::ALL) ? format : nil
       end
 
+      def should_apply_vary_header?
+        !params_readable? && use_accept_header && valid_accept_header
+      end
+
       private
         BROWSER_LIKE_ACCEPTS = /,\s*\*\/\*|\*\/\*\s*,/
 
+        def params_readable? # :doc:
+          parameters[:format]
+        rescue *RESCUABLE_MIME_FORMAT_ERRORS
+          false
+        end
+
         def valid_accept_header # :doc:
           (xhr? && (accept.present? || content_mime_type)) ||
             (accept.present? && accept !~ BROWSER_LIKE_ACCEPTS)