oh sorry, didn't see this: https://github.com/code-mancers/invoker/pull/35/files#diff-ebcdca612b41d4f031db66e6cf460842R148
but still why so many if conditions? why can't we do this:

```ruby
  header.slice(*required_fields).each do |k, v|
    final_string << "#{k}: #{v}"
  end

  final_string.join("\r\n") + "\r\n\r\n" + body 
```

i might be wrong.