Use lastPos ONLY for tracking what has been .slice()'d, never as an
indication of if key/value has been seen, since lastPos is updated on
seeing + as well.
PR-URL: https://github.com/nodejs/node/pull/14151
Fixes: https://github.com/nodejs/node/issues/13773
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>