Request body: do not create temp file if there's nothing to write.

Request body: do not create temp file if there's nothing to write.

Do this only when the entire request body is empty and
r->request_body_in_file_only is set.

The issue manifested itself with missing warning "a client request body is
buffered to a temporary file" when the entire rb->buf is full and all buffers
are delayed by a filter.