Multiple COOKIE headers are not allowed by RFC 6265

[paolovolpi]

Method Request::append_cookie_headers add multiple COOKIE headers to a request while having more then one cookie set but this is not allowed by RFC 6265.:

5.4. The Cookie Header

When the user agent generates an HTTP request, the user agent MUST
NOT attach more than one Cookie header field.

The correct behavior would be to set a single COOKIE header with the following syntax:

4.2.1. Syntax
cookie-header = "Cookie:" OWS cookie-string OWS
cookie-string = cookie-pair *( ";" SP cookie-pair )