@Test
  void headerMap() throws Exception {
    server.enqueue(new MockResponse());

    TestInterface api = new TestInterfaceBuilder().target("http://localhost:" + server.getPort());

    Map<String, Object> headerMap = new LinkedHashMap<>();
    headerMap.put("Content-Type", "myContent");
    headerMap.put("Custom-Header", "fooValue");
    api.headerMap(headerMap);

    assertThat(server.takeRequest())
        .hasHeaders(
            entry("Content-Type", Arrays.asList("myContent")),
            entry("Custom-Header", Arrays.asList("fooValue")));
  }
The type of a parameter value may not be the desired type to encode as. Prefer the interface
type.
core/src/test/java/feign/FeignTest.java