http_send: Send http OK message earlier.

Without this change, the http OK message is sent just when the vss is about to start
streaming, i.e. at the end of the announce interval. This might be too late and sometimes
causes the client to not receive the first chunk(s).

Fix this bug by adding the client file descriptor to the write fd set in http_preselect()
in case we received the get request (or something else) so that the following call
to select() will return immediately. The http OK message (or an error message) will
then be sent much earlier.