xff request setting default should be leave

[jrynyt]

If you add a request_setting but do not specify an xff parameter, it defaults to append, which actually inserts vcl to change X-Forwarded-For even if your request setting has nothing to do with that. Or if you have multiple request settings, one of which specifies xff, a subsequent request setting that doesn't include xff will override it.

The default should be leave, which does nothing. The fastly API reference does not specify a default: https://docs.fastly.com/api/config#request_settings

[sjparkinson]

This is somewhat related to #489, as re-creating the Force TLS setting currently needs a block like:

request_setting {
  name          = "Generated by force TLS and enable HSTS"
  force_ssl     = true
  xff           = ""
}