val headers = headersOf("a", "b", "c", "d")
    assertThat(headers["a"]).isEqualTo("b")
    assertThat(headers["c"]).isEqualTo("d")
    assertThat(headers["e"]).isNull()
  }

  @Test fun iteratorOperator() {
    val headers = headersOf("a", "b", "c", "d")

    val pairs = mutableListOf<Pair<String, String>>()
    for ((name, value) in headers) {
      pairs += name to value
    }