Content-Encoding header is not being properly parsed

[mmukahhal-tibco]

Content encodings can be supplied by multiple Content-Encoding headers or by one header with a comma separated list of encodings as seen here.

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.