ResponseCaching returns no headers in 304 NotModified response

Describe the bug

ResponseCaching returns no headers in 304 NotModified response

To Reproduce

Steps to reproduce the behavior:

1. Setup a server that uses ResponseCaching
2. Make a request that responds with a Last-Modified and Expires header and see that it gets put into the ResponseCache
3. The Response will be a 200 with headers including Last-Modified, Expires, Cache-Control
4. Make the same request again, but add the If-Modified-Since header with a value the same as the previously returned Last-Modified time
5. The Response will be a 304 but it won’t have any headers.

Expected behavior

The 304 response should contain a number of headers as specified in RFC 7232

https://tools.ietf.org/html/rfc7232#section-4.1

   The server generating a 304 response MUST generate any of the
   following header fields that would have been sent in a 200 (OK)
   response to the same request: Cache-Control, Content-Location, Date,
   ETag, Expires, and Vary.

Additional context

You can see this quite clearly in https://github.com/aspnet/AspNetCore/blob/master/src/Middleware/ResponseCaching/src/ResponseCachingMiddleware.cs#L156

[alefranz]

I will give it a go

[analogrelay]

Indeed! Unfortunately “Addresses” doesn’t close the issue out when the PR is closed 😃.