Case-sensitive Set-Cookie parsing

[vlm]

The current parsing is case-sensitive:

https://github.com/swift-server/async-http-client/blob/master/Sources/AsyncHTTPClient/HTTPClient%2BHTTPCookie.swift#L91

Need to make it case-insensitive as per https://tools.ietf.org/html/rfc6265#section-5.2.2