more easily get access to headers in Easy2 client

In the Easy struct there is a `header_function` (https://docs.rs/curl/0.4.44/curl/easy/struct.Easy.html#method.header_function) that allows one to get access to all headers. This option is however not available to `Easy2`.

Would it be possible to expose this somehow? as currently I need to use something very hacky. I use `show_headers` and parse it myself from the body... Which feels like a pretty wrong approach of having to do stuff and currently only works because I am focussing on a fairly easy HTTP/1.1 usage.