Multiple `@CookieValue` client arguments generates multiple `Cookie` headers

[jameskleeh]

Issue description

According to https://www.rfc-editor.org/rfc/rfc6265#section-5.4 there must only be a single header

This restriction is relaxed for HTTP/2

https://www.rfc-editor.org/rfc/rfc7540#section-8.1.2.5