Support multiple headers with the same header name

[rylev]

HTTP headers can be duplicated where a single logical header name has multiple values. This is why wasi-http returns a list of header values when you query for a given header name. The SDK papers over this and takes the last value it sees as the only value for the header.

Unfortunately solving this issue would be a breaking change as the SDK exposes this fact to users. For example, here.

[lann]

You can almost fix this with normalization: https://httpwg.org/http-core/draft-ietf-httpbis-semantics-latest.html#fields.order

Unfortunately set-cookies cannot be normalized. I suppose we could add a separate accessor for them?