Save pre-compressed body for logging

[UnbrandedTech]

Right now, I have to choose between compressing my responses and logging them...

I tried to go down the road of unzipping/inflating them after, but decided that maybe the library could help out a bit.

I purpose something along the lines of adding a new prop onto the response object called pre_compressed_body or something similar so if someone wants to log the response they can.

[jonathanong]

this can be done with another middleware after compress in the stack. put something like this after the compress middleware https://github.com/koajs/logger/blob/6bec1297e3040c2c90724bda716e2c66bfad303e/index.js#L77-L100