Remove serializer::keep_alive (API Change):

* The serializer::keep_alive function is removed

Actions Required:

* Call message::keep_alive instead of serializer::keep_alive

include/boost/beast/http/impl/serializer.ipp

@@ -77,7 +77,6 @@ next(error_code& ec, Visit&& visit)
     {
         frdinit(std::integral_constant<bool,
             isRequest>{});
-        keep_alive_ = m_.keep_alive();
         chunked_ = m_.chunked();
         if(chunked_)
             goto go_init_c;