I have installed the development environment and implemented keep-alive for HttpResponder.
Its has no impact on browser calls, those sockets get closed even when Connection: keep-alive is set.
If the Rrf-Keep-Alive: force header is set, keep-alive starts working for a single socket. (so max 1 concurrent open socket)
__LPC17xx__ is also checked and, if set, disables keep-alive.
I have to do some more testing, but all seems to work smooth.
If interested I can create a pull request, for you to verify?
Cheers
Gerd