@Test
    void sendRequestWithEmptyContent() throws Exception {
        try (ServerSocket ss = new ServerSocket(0)) {
            final int port = ss.getLocalPort();
            final WebClient client = WebClient.of("h1c://127.0.0.1:" + port);
            final CompletableFuture<AggregatedHttpResponse> future =
                    client.prepare()
                          .get("/")
                          .header(HttpHeaderNames.EXPECT, HttpHeaderValues.CONTINUE)
                          .execute()
                          .aggregate();

            assertThatThrownBy(future::join)
                    .hasCauseInstanceOf(UnprocessedRequestException.class)
                    .cause()
                    .hasCauseInstanceOf(IllegalArgumentException.class)
                    .hasMessageContaining("Expect: 100-continue header");
        }
    }
This test is to check the behavior of the HttpClient when the 'Expect: 100-continue' header is set.
core/src/test/java/com/linecorp/armeria/client/HttpClientExpect100HeaderTest.java