change raw-headers to response-headers

`<response>` has no slot `raw-headers`

d3320d53c5f5443c5a75f56ebbc0935833fc04c3 changed an instance of `raw-headers` to `response-headers`, it seems like this should be the same.

Caused an issue with `debug: #t` on `<http-server>` since the getter was undefined.
Merge pull request #62 from kibook/patch-4

change raw-headers to response-headers