@Test
  public void testContentTypeWithoutCharset() throws Exception {
    server.enqueue(new MockResponse().setBody("AAAAAAAA"));
    OkHttpClientTestInterface api =
        newBuilder()
            .target(OkHttpClientTestInterface.class, "http://localhost:" + server.getPort());

    Response response = api.getWithContentType();
    // Response length should not be null
    assertThat(Util.toString(response.body().asReader(Util.UTF_8))).isEqualTo("AAAAAAAA");

    MockWebServerAssertions.assertThat(server.takeRequest())
        .hasHeaders(
            MapEntry.entry("Accept", Collections.singletonList("text/plain")),
            MapEntry.entry("Content-Type", Collections.singletonList("text/plain")))
        .hasMethod("GET");
  }
Tests client-specific behavior, such as ensuring Content-Length is sent when specified.
okhttp/src/test/java/feign/okhttp/OkHttpClientTest.java