Match trusted proxies as IP addresses

set_trusted_proxies() compared addresses as strings, so a proxy listed
as 10.0.0.2 was not trusted when a server listening on both IPv4 and
IPv6 saw it as ::ffff:10.0.0.2, and two spellings of one IPv6 address
did not match either. The listed addresses are now put in one spelling
when they are set, and so are the connection peer and each
X-Forwarded-For hop before they are looked up; an IPv4-mapped IPv6
address becomes its IPv4 address. The README now documents the setter.

test/test.cc

@@ -22562,6 +22562,55 @@ TEST(ForwardedHeadersTest, NoForwardedHeaders) {
               observed_remote_addr == "127.0.0.1");
 }
 
+// A server listening on both IPv4 and IPv6 sees an IPv4 peer as an
+// IPv4-mapped IPv6 address, which still matches the proxy's IPv4 address
+TEST(ForwardedHeadersTest, IPv4MappedPeerMatchesIPv4Proxy) {
+  Server svr;
+  svr.set_trusted_proxies({"127.0.0.1"});
+  svr.Get("/ip", [](const Request &req, Response &res) {
+    res.set_content(req.remote_addr, "text/plain");
+  });
+
+  auto port = svr.bind_to_any_port("::");
+  if (port <= 0) { GTEST_SKIP() << "no IPv6 on this host"; }
+  thread t = thread([&] { svr.listen_after_bind(); });
+  auto se = detail::scope_exit([&] {
+    svr.stop();
+    t.join();
+    ASSERT_FALSE(svr.is_running());
+  });
+  svr.wait_until_ready();
+
+  Client cli("127.0.0.1", port);
+  auto res = cli.Get("/ip", Headers{{"X-Forwarded-For", "198.51.100.23"}});
+  ASSERT_TRUE(res) << "Error: " << to_string(res.error());
+  EXPECT_EQ("198.51.100.23", res->body);
+}
+
+TEST(ForwardedHeadersTest, IPv4MappedHopMatchesIPv4Proxy) {
+  Server svr;
+  svr.set_trusted_proxies({"192.0.2.45", "::1", "127.0.0.1"});
+  svr.Get("/ip", [](const Request &req, Response &res) {
+    res.set_content(req.remote_addr, "text/plain");
+  });
+
+  auto port = svr.bind_to_any_port(HOST);
+  ASSERT_GT(port, 0);
+  thread t = thread([&] { svr.listen_after_bind(); });
+  auto se = detail::scope_exit([&] {
+    svr.stop();
+    t.join();
+    ASSERT_FALSE(svr.is_running());
+  });
+  svr.wait_until_ready();
+
+  Client cli(HOST, port);
+  auto res = cli.Get(
+      "/ip", Headers{{"X-Forwarded-For", "198.51.100.23, ::ffff:192.0.2.45"}});
+  ASSERT_TRUE(res) << "Error: " << to_string(res.error());
+  EXPECT_EQ("198.51.100.23", res->body);
+}
+
 TEST(ForwardedHeadersTest, SingleTrustedProxy_UsesIPBeforeTrusted) {
   Server svr;