Resolve #2017

Author: yhirose

httplib.h

@@ -628,6 +628,7 @@ struct Request {
   Ranges ranges;
   Match matches;
   std::unordered_map<std::string, std::string> path_params;
+  std::function<bool()> is_connection_closed = []() { return true; };
 
   // for client
   ResponseHandler response_handler;
@@ -7217,6 +7218,11 @@ Server::process_request(Stream &strm, const std::string &remote_addr,
     }
   }
 
+  // Setup `is_connection_closed` method
+  req.is_connection_closed = [&]() {
+    return !detail::is_socket_alive(strm.socket());
+  };
+
   // Routing
   auto routed = false;
 #ifdef CPPHTTPLIB_NO_EXCEPTIONS