Combine repeated response headers by concatenating their values with a comma separator

The spec isn't exactly inspiring in its clarity, but I'm pretty sure [this section](http://tools.ietf.org/html/draft-ietf-httpbis-http2-10#section-8.1.3.3) allows (or even requires?) this concatenation. Special handling for `set-cookie`/`cookie` can be added later, in the `h2-10` branch.