Remove Option from RequestHeadType::Rc

When working with middlewares, I needed to destructure into the ConnectRequest and RequestHeadType to add new headers into the request. If `RequestHeadType::Rc` defaulted to having an empty headermap instead of an Option it would be possible to implement a `headers_mut()` method on ConnectRequest for easy header additions.

I realize that providing a `headers_mut()` for the `RequestHeadType::Rc` case might be misleading, since it won't be possible to remove existing headers, but maybe with a bit of documentation that could be fine