private TestInterface buildTestInterface(boolean compress) {
    return newBuilder()
        .requestInterceptor(req -> req.header("Content-Encoding", compress ? "gzip" : ""))
        .target(TestInterface.class, "http://localhost:" + server.getPort());
  }
Tests that 'Content-Encoding: gzip' is handled correctly
hc5/src/test/java/feign/hc5/GzipHttp5ClientTest.java