assertThat(body.contentLength()).isEqualTo(-1)
    val buffer = Buffer()
    body.writeTo(buffer)
    assertThat(buffer.readUtf8()).isEqualTo(expected)
  }

  @Test
  fun contentTypeHeaderIsForbidden() {
    val multipart = MultipartBody.Builder()
    assertFailsWith<IllegalArgumentException> {
      multipart.addPart(
        headersOf("Content-Type", "text/plain"),