@Suppress("UNCHECKED_CAST")
interface HasHeaders <out T> {
    val headers: MutableList<HeaderPair>

    fun getHeader(key: Header): String? {
        return headers.find { it.first == key }?.second
    }

    // if value is null, then the header does not get set
    fun setHeader(key: Header, value: String?): T {
        if (value == null) return this as T
        removeHeader(key)
        appendHeader(key, value)
        return this as T
    }

    // if value is null, then the header does not get set
    fun appendHeader(key: Header, value: String?): T {
        if (value == null) return this as T
        headers.add(key to value)
        return this as T
    }

    fun removeHeader(key: Header): T {
        headers.removeIf { it.first == key }
        return this as T
    }
}

class Request(): HasHeaders<Request>
class Response(): HasHeaders<Response>