Fix cqueues adapter: support multi-value response headers (Set-Cookie)

[39george]

This PR fixes a crash in the cqueues (lua-http) backend when an action sets more than one cookie.

Lapis stores multiple Set-Cookie values as an array in res.headers, but the cqueues adapter was appending header values directly and passed a Lua table to http.headers:append, causing field value invalid. The adapter now flattens table-valued headers and appends each value separately, allowing multiple cookies (and any multi-value header) to work correctly.

Repro: set self.cookies.csrf and self.cookies.theme alongside self.session.* → server crashes.
After: multiple Set-Cookie headers are emitted correctly