Entity tags: explicit flag to skip not modified filter. Previously, last_modified_time was tested against -1 to check if the not modified filter should be skipped. Notably, this prevented nginx from additional If-Modified-Since (et al.) checks on proxied responses. Such behaviour is suboptimal in some cases though, as checks are always skipped on responses from a cache with ETag only (without Last-Modified), resulting in If-None-Match being ignored in such cases. Additionally, it was not possible to return 412 from the If-Unmodified-Since if last modification time was not known for some reason. This change introduces explicit r->disable_not_modified flag instead, which is set by ngx_http_upstream_process_headers().