Add Content-Location header

[Stebalien]

See https://tools.ietf.org/html/rfc7231#section-3.1.4.2

[koivunej]

I'd like to take a shot at this. ABNF from RFC here is:

Content-Location = absolute-URI / partial-URI

ABNF is exactly same as with Referer which is implemented as

header! {
    // TODO Use URL
    (Referer, "Referer") => [String]

    // testcase
}

/TODO:? Use URL/ is mentioned for Location and Referer headers, so I guess it'd be best to implement Content-Location as simple (ContentLocation, "Content-Location") => [String] for consistency?