server: support dual-stack server socket (#450)

According to RFC 3493 the socket option IPV6_V6ONLY
should be off by default, see
https://tools.ietf.org/html/rfc3493#page-22 (chapter 5.3).

However this does not seem to be the case on all systems.
For instance on any Windows OS, the option is on by default.

Therefore clear this option in order to allow
an server socket which can support IPv6 and IPv4 at the same time.

httplib.h

@@ -1469,6 +1469,11 @@ socket_t create_socket(const char *host, int port, Fn fn,
     int yes = 1;
     setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, reinterpret_cast<char *>(&yes),
                sizeof(yes));
+
+    int no = 0;
+    setsockopt(sock, IPPROTO_IPV6, IPV6_V6ONLY, reinterpret_cast<char *>(&no),
+               sizeof(no));
+
 #ifdef SO_REUSEPORT
     setsockopt(sock, SOL_SOCKET, SO_REUSEPORT, reinterpret_cast<char *>(&yes),
                sizeof(yes));

test/test.cc

@@ -724,6 +724,39 @@ TEST(RedirectToDifferentPort, Redirect) {
 }
 #endif
 
+TEST(Server, BindDualStack) {
+  Server svr;
+
+  svr.Get("/1", [&](const Request & /*req*/, Response &res) {
+    res.set_content("Hello World!", "text/plain");
+  });
+
+  auto thread = std::thread([&]() { svr.listen("::", PORT); });
+
+  // Give GET time to get a few messages.
+  std::this_thread::sleep_for(std::chrono::seconds(1));
+
+  {
+    Client cli("127.0.0.1", PORT);
+
+    auto res = cli.Get("/1");
+    ASSERT_TRUE(res != nullptr);
+    EXPECT_EQ(200, res->status);
+    EXPECT_EQ(res->body, "Hello World!");
+  }
+  {
+    Client cli("::1", PORT);
+
+    auto res = cli.Get("/1");
+    ASSERT_TRUE(res != nullptr);
+    EXPECT_EQ(200, res->status);
+    EXPECT_EQ(res->body, "Hello World!");
+  }
+  svr.stop();
+  thread.join();
+  ASSERT_FALSE(svr.is_running());
+}
+
 TEST(Server, BindAndListenSeparately) {
   Server svr;
   int port = svr.bind_to_any_port("0.0.0.0");