When a request has multiple values for a same header, the response combine them into a CSV. For example:

GET :target/echo/hello/world?a=10&b=20
x-h: 10
x-h: 20
x-h: 30
x-h: 1, 2, 3

The above request will produce the following response:

{
  ...
  "headers": {
    "x-h": "10, 20, 30, 1, 2, 3",
    ...

This makes it impossible to know if the header in the request was CSV or had multiple values. Improve this to return like the following:

{
  "headers": {
    "x-h": [
      "10",
      "20",
      "30",
      "1, 2, 3"
    ],
    ...