Prior to this commit, `HttpEntityMethodProcessor` would rely on
`ServletWebRequest` to process conditional requests and with incoming
`"If-Modified-Since"` / `"If-None-Match"` request headers.
This approach is problematic since in that class:
* response is wrapped in a `ServletServerHttpResponse`
* this wrapped response does not write response headers right away
* `ServletWebRequest.checkNotModified` methods can't apply their
logic with incomplete response headers
This solution adds some minimal code duplication and applies
the conditional request logic within the Processor.
A possible alternative would be to improve the
`ServletServerHttpResponse$ServletResponseHttpHeaders` implementation
with write methods - but this solution would only work for Servlet 3.x
applications.
Issue: SPR-13090
39d689da0c