Rename to serializer::keep_alive (API Change):

The serializer member is renamed to keep_alive()

Actions Required:

* Use serializer::keep_alive instead of serializer::close and
  take the logical NOT of the return value.

include/beast/http/impl/serializer.ipp

@@ -86,7 +86,7 @@ next(error_code& ec, Visit&& visit)
     {
         frdinit(std::integral_constant<bool,
             isRequest>{});
-        close_ = ! frd_->keep_alive();
+        keep_alive_ = frd_->keep_alive();
         chunked_ = frd_->chunked();
         if(chunked_)
             goto go_init_c;