call `write_core` when there is pending data in sendvec
call `write_core` when there is pending data in sendvec

h2o/h2o,devnexen/h2o,h2o/h2o,devnexen/h2o,h2o/h2o,devnexen/h2o,h2o/h2o,h2o/h2o,h2o/h2o,devnexen/h2o,devnexen/h2o,devnexen/h2o,h2o/h2o,devnexen/h2o,h2o/h2o,devnexen/h2o,devnexen/h2o,h2o/h2o