Description
Steps to reproduce the problem:
- cache is empty
- send a conditional If-modified-since request
- backend server responds with a 304 Not modified (the response does not contain the "Last-modified" header, this conforms to rfc 2616 chapter 10.3.5)
- the response is stored in the cache
- send another conditional If-modified-since request
- as the cache entry does not contain a Last-modified header to match with the If-modified-since request header, the cache considers the entry cannot be used and sends a request again to the backend server