server: close all connections correctly at shutdown

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

lib/fluent/plugin_helper/server.rb

@@ -356,7 +356,7 @@ def stop
       end
 
       def shutdown
-        @_server_connections.each do |conn|
+        @_server_connections.dup.each do |conn|
           conn.close rescue nil
         end