There's a small error in the way the extension parses the configuration as JSON. It's adding quotes to them.

Example (note the header-one and another-header values):

$ curl -I localhost:18000/
HTTP/1.1 200 OK
content-length: 13
content-type: text/plain
another-header: "another value"
header-one: "value one"
date: Thu, 17 Jun 2021 17:19:56 GMT
server: envoy

While fixing this, let's also add a test so that we don't get regressions.