Upstream: fixed error handling of stale and revalidated cache send. The NGX_DONE value returned from ngx_http_upstream_cache_send() indicates that upstream was already finalized in ngx_http_upstream_process_headers(). It was treated as a generic error which resulted in duplicate finalization. Handled NGX_HTTP_UPSTREAM_INVALID_HEADER from ngx_http_upstream_cache_send(). Previously, it could return within ngx_http_upstream_finalize_request(), and since it's below NGX_HTTP_SPECIAL_RESPONSE, a client connection could stuck.