fun tagsAreImmutable() {
    val builder =
      Request.Builder()
        .url("https://square.com")
    val requestA = builder.tag(String::class.java, "a").build()
    val requestB = builder.tag(String::class.java, "b").build()
    val requestC = requestA.newBuilder().tag(String::class.java, "c").build()
    assertThat(requestA.tag(String::class.java)).isSameAs("a")
    assertThat(requestB.tag(String::class.java)).isSameAs("b")