Reduce the transformations needed on header fields, so they can be more often
set directly and no need to split and recombine.
The Content-Length field is added IF it is needed for framing or if it was explicitly set
The Transfer-Encoding: chunk field is used only as a hint that there is content.
Connection fields are used as is, but are checked for close and keep-alive
Greg Wilkins
Simone Bordet
Joakim Erdfelt