@Test
  void headersOnTypeAddsContentTypeHeader() throws Exception {
    final MethodMetadata md = parseAndValidateMetadata(HeadersOnType.class, "post");

    assertThat(md.template())
        .hasHeaders(
            entry("Content-Type", asList("application/xml")),
            entry("Content-Length", asList(String.valueOf(md.template().body().length))));
  }
Tests interfaces defined per {@link Contract.Default} are interpreted into expected {@link feign
.RequestTemplate template} instances.
core/src/test/java/feign/DefaultContractTest.java