Allow Localization concern to send Content-Language

[Kaliumhexacyanoferrat]

As a client, I would like to know the actual language/locale that was used by the server to generate my content.

Example

By default, the server will send the Content-Language that was negotiated (only if there was an Accept-Language), but users may opt out:

Localization.Create()
            .SendLanguage(false);

Acceptance criteria

• If the requests contains an Accept-Language header, the server will generate a Content-Language header in response
• The feature is documented on the GenHTTP website
• The feature is covered by acceptance tests

[Kaliumhexacyanoferrat]

Just send it always and also when it is not read from the Accept-Language header ...