silence warning

@bf4

```
spec/mail/network/delivery_methods/file_delivery_spec.rb:89: warning: File.exists? is a deprecated name, use File.exist? instead
spec/mail/network/delivery_methods/file_delivery_spec.rb:74: warning: File.exists? is a deprecated name, use File.exist? instead
spec/spec_helper.rb:33: warning: setting Encoding.default_external
```