implement infestor test and logic

- no longer skip infestor test
- assert a few more things in infestor test
- test now has `</html>` to injectAt
- we disable gzip if you attempt to write the res

jeff and I discussed this a little bit and we basically landed on the fact that if you want to write to the response then you can't do that when gzip is on. along with this, `alchemist` turns off gzip by default but I've also manually disabled gzip if you pass a `write` option. i've also made note in the readme of that quirk. 

/cc @jenius @joshrowley