Fix HTTP server write (1v85) regression (fix #769)
Fix HTTP server write (1v85) regression (fix #769)

When sending all data, put an empty string in the `dSnd` field
if all data is used. This shows that headers have already been
sent.

It's actually marginally faster/more efficient than the old
method, which used a separate var to flag whether headers had
been sent or not.

luetgendorf/Espruino,jasonkajita/Espruino,luetgendorf/Espruino,mjdietzx/Espruino,luetgendorf/Espruino,luetgendorf/Espruino,lancernet/Espruino,tve/Espruino,mjdietzx/Espruino,muet/Espruino,mjdietzx/Espruino,jasonkajita/Espruino,tve/Espruino,redbear/Espruino,nkolban/Espruino,nkolban/Espruino,redbear/Espruino,tve/Espruino,jasonkajita/Espruino,wilberforce/Espruino,nkolban/Espruino,jasonkajita/Espruino,wilberforce/Espruino,mjdietzx/Espruino,wilberforce/Espruino,muet/Espruino,lancernet/Espruino,redbear/Espruino,mjdietzx/Espruino,nkolban/Espruino,lancernet/Espruino,nkolban/Espruino,muet/Espruino,muet/Espruino,tve/Espruino,jasonkajita/Espruino,mjdietzx/Espruino,tve/Espruino,luetgendorf/Espruino,mjdietzx/Espruino,redbear/Espruino,wilberforce/Espruino,muet/Espruino,wilberforce/Espruino,nkolban/Espruino,lancernet/Espruino,wilberforce/Espruino,redbear/Espruino,muet/Espruino,lancernet/Espruino,luetgendorf/Espruino,wilberforce/Espruino,jasonkajita/Espruino,luetgendorf/Espruino,lancernet/Espruino,tve/Espruino,nkolban/Espruino,muet/Espruino,lancernet/Espruino,redbear/Espruino,tve/Espruino,redbear/Espruino,jasonkajita/Espruino