Fix #430

* When use limit_rate in nginx.conf, wev->delayed will be set if needed. We shoud set wev->delayed to 0 when delete the delayed timer in `ngx_http_lua_flush_pending_output`. It will hang the outout forever otherwise because the `ngx_http_write_filter` will return before outout if wev->delaye = 1. 

* ngx.flush(true) should block until all the delayed data has been sent, or ngx.flush(true) will be useless with limit_rate being set.