http2 compat is missing writableFinished for OutgoingMessage.

I tried fixing this myself but the whole finished flow for http2 compat confuses me.

• finished seems to be looking at a lot of different stuff (e.g. what does abort have to do with finish?).
• 'finish' is emitted on stream.on('close', ...).
• Sometimes we listen to this.on('finish', ...) and sometimes stream.on('finish', ...).

I think someone with a clearer understanding needs to look at this.