adjust offset when sending data in the middle of send buffer
adjust offset when sending data in the middle of send buffer

yannick/h2o,devnexen/h2o,h2o/h2o,h2o/h2o,h2o/h2o,h2o/h2o,devnexen/h2o,yannick/h2o,yannick/h2o,cwyang/h2o,devnexen/h2o,cwyang/h2o,devnexen/h2o,devnexen/h2o,yannick/h2o,cwyang/h2o,yannick/h2o,h2o/h2o,devnexen/h2o,cwyang/h2o,devnexen/h2o,cwyang/h2o,h2o/h2o,yannick/h2o,yannick/h2o,yannick/h2o,cwyang/h2o,h2o/h2o,devnexen/h2o,cwyang/h2o,yannick/h2o,h2o/h2o,h2o/h2o,devnexen/h2o,cwyang/h2o