Note that clients can actually send three types of messages - requests, ACKs of previous pushes, and NACKs of previous pushes. Unfortunately, these are not clearly distinguished in the API, so there is some logic to split these out (`shouldRespond`).

#### Pushes

     */
    fun addPush(promise: PushPromise) =
      apply {
        this.pushPromises += promise
      }

    /**
     * When [protocols][MockWebServer.protocols] include [HTTP_2][okhttp3.Protocol], this pushes
     * [settings] before writing the response.
     */
    fun settings(settings: Settings) =
      apply {
        this.settings.clear()
        this.settings.merge(settings)
      }

    /**