BUG/MEDIUM: http-client: Notify applet has more data to deliver until the EOM

BUG/MEDIUM: http-client: Notify applet has more data to deliver until the EOM

When we leave the I/O handler with an unfinished request, we must report the
applet has more data to deliver. Otherwise, when the channel request buffer
is emptied, the http-client applet is not always woken up to forward the
remaining request data.

This issue was probably revealed by commit "BUG/MEDIUM: http-client: Don't
wake http-client applet if nothing was xferred". It is only an issue with
large POSTs, when the payload is streamed.

This patch must be backported as far as 2.6 with the commit above. But on
older versions, the applet API may differ. So be careful.