message: Clean the response headers when sending kNotInCache
  When we send out a 501 in the IPRO path for a http cache
  hit, clear the response headers so we do not leave
  behind stray headers.
  Fixes
diff: @@ -139,6 +139,10 @@ void RecordingFetch::HandleHeadersComplete() {
  // kNotInCacheStatus instead to fall back to the server's native method of
  // serving the url and indicate we do want it recorded.
  if (!response_headers()->IsErrorStatus()) {
  + // Clear the response headers to ensure stray headers do not end up
  + // being put on the wire:
  + // https://github.com/pagespeed/mod_pagespeed/issues/1496
  + response_headers()->Clear();
  response_headers()->set_status_code(
  CacheUrlAsyncFetcher::kNotInCacheStatus);
  }
  