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

    Response response = api.postWithContentType("foo", "text/plain");
    // Response length should not be null
    assertThat(Util.toString(response.body().asReader(UTF_8))).isEqualTo("AAAAAAAA");
  }
Some client implementation tests should override this test if the PATCH operation is
unsupported.
core/src/test/java/feign/client/AbstractClientTest.java