Terminate the chunked printDirectory (#6808)

Fixes #2481

Send a 0-len chunk when the directory is completed (in a chunked HTTP
transfer) to terminate the HTTP transfer properly.

Author: earlephilhower

libraries/ESP8266WebServer/examples/SDWebServer/SDWebServer.ino

@@ -241,6 +241,7 @@ void printDirectory() {
     entry.close();
   }
   server.sendContent("]");
+  server.sendContent(""); // Terminate the HTTP chunked transmission with a 0-length chunk
   dir.close();
 }