Oops, I modified with windows, I forgotten the indentation...

I succeed the replacement with this example :

```
filter {
  noop {
    add_field => [ "test_message", "Test<n>" ]
  }
  mutate {
    gsub => [ "test_message", "<n>", "\\n" ] {
  }
}

email {
  body => "%{test_message}"
  ...
}
```

Problem when I use /\n+/ with "\n" in the gsub mutate.