net: lib: http_server: Respect Connection close header

In case no "Connection: close" header is present in the request, the
server should keep the connection open for the client. Hence, after
serving a request, we need to check if the header was present (the
parser sets a flag for it), and only close the connection immediately,
if the client requested it. In case the client remains silent, the
connection will be closed anyway after the inactivity timer kicks in.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>