Render Cookies as cookie-pairs in Cookie-headers

see http://tools.ietf.org/search/rfc6265#section-4.2

Currently all cookie attributes are rendered into a Cookie-header.

This patch makes the rendering symmetric to the [parsing](https://github.com/spray/spray/blob/master/spray-http/src/main/scala/spray/http/parser/CookieHeaders.scala#L32-L38).