@Test
  void useCaseInsensitiveHeaderFieldNames() {
    final RequestTemplate template = new RequestTemplate();

    final String value = "value1";
    template.header("TEST", value);

    final String value2 = "value2";
    template.header("tEST", value2);

    final Collection<String> test = template.headers().get("test");

    final String assertionMessage = "Header field names should be case insensitive";

    assertThat(test).as(assertionMessage).isNotNull();
    assertThat(test.contains(value)).as(assertionMessage).isTrue();
    assertThat(test.contains(value2)).as(assertionMessage).isTrue();
    assertThat(template.headers()).hasSize(1);
    assertThat(template.headers().get("tesT")).hasSize(2);
  }
This ensures we don't mess up vnd types
core/src/test/java/feign/RequestTemplateTest.java