.protocols(listOf(Protocol.H2_PRIOR_KNOWLEDGE))
        .build()
    server.useHttps(handshakeCertificates.sslSocketFactory())
    server.enqueue(MockResponse())
    val call = client.newCall(Request(server.url("/")))
    assertFailsWith<UnknownServiceException> {
      call.execute()
    }.also { expected ->
      assertThat(expected.message).isEqualTo("H2_PRIOR_KNOWLEDGE cannot be used with HTTPS")
    }
  }

    code][https_server_sample].

 *  **OkHttp now supports prior knowledge cleartext HTTP/2.** Enable this by setting
    `Protocol.H2_PRIOR_KNOWLEDGE` as the lone protocol on an `OkHttpClient.Builder`. This mode
    only supports `http:` URLs and is best suited in closed environments where HTTPS is
    inappropriate.