Content-Encoding header is not being properly parsed

Content encodings can be supplied by multiple `Content-Encoding` headers or by one header with a comma separated list of encodings as seen [here](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Encoding).

The `Header` map embedded in an `http.Request` from the Go standard library states that:

```
Header contains the request header fields either received by the server or to be sent by the client.

If a server received a request with header lines,

Host: example.com
accept-encoding: gzip, deflate
Accept-Language: en-us
fOO: Bar
foo: two
then

Header = map[string][]string{
"Accept-Encoding": {"gzip, deflate"},
"Accept-Language": {"en-us"},
"Foo": {"Bar", "two"},
}
```

As seen by the `Accept-Encoding` header, this means a comma separated list of values will be returned as a `[]string` with a _single_ element.

Currently, `chi` does not make this distinction, and so it is not properly parsing the `Content-Encoding` header.

I will submit a PR fixing this issue shortly.

Start by locating chi's handling of the Content-Encoding header and compare it with Go's http.Header behavior described in the issue. Check how repeated headers and comma-separated values are represented, then add coverage for both forms; done means both encodings are parsed correctly.

Written by the indexing model from the issue text.