Encode the characters RFC 3986 doesn't allow in a client path

encode_path() sent '"', '<', '>', '\', '^', '`', '{', '|', '}', '['
and ']' raw, although RFC 3986 §3.3 doesn't allow them in a path.
Tomcat, Jetty and Undertow answer each of them with 400 by default, but
accept the percent-encoded form. It also stopped at an embedded NUL, so
"/a\0b" went out as "/a". Encode every byte outside pchar, keeping '/'
and '%' so that escapes pass through, and still encode '+', ',', ';'
and '\'' as before.

encode_request_target() now passes only the path to encode_path(). The
"http://host" of a target sent to a proxy and a CONNECT "host:port" are
sent as they are, so the brackets of an IPv6 host stay.

test/test.cc

@@ -4457,6 +4457,10 @@ TEST(PathUrlEncodeTest, StreamingMatchesBufferedTarget) {
       {"/a b?x=1", "/a%20b?x=1"},
       {"/\xE6\x97\xA5\xE6\x9C\xAC", "/%E6%97%A5%E6%9C%AC"},
       {"/a,b;c+d", "/a%2Cb%3Bc%2Bd"},
+      // Characters RFC 3986 doesn't allow in a path.
+      {"/\"<>\\^`{|}[]", "/%22%3C%3E%5C%5E%60%7B%7C%7D%5B%5D"},
+      // Escapes and the rest of pchar are kept.
+      {"/a%20b/!$&()*=:@~", "/a%20b/!$&()*=:@~"},
   };
 
   for (const auto &c : cases) {
@@ -4572,9 +4576,57 @@ TEST(PathUrlEncodeTest, ControlCharsInPathAreEncoded) {
     auto res = cli.Get("/a\tb\x1b\x7f");
     ASSERT_TRUE(res) << "Error: " << to_string(res.error());
     EXPECT_EQ("/a%09b%1B%7F", target);
+
+    // An embedded NUL doesn't cut the path short.
+    res = cli.Get(std::string("/a\0b", 4));
+    ASSERT_TRUE(res) << "Error: " << to_string(res.error());
+    EXPECT_EQ("/a%00b", target);
   }
 }
 
+TEST(PathUrlEncodeTest, HostInProxyTargetIsNotEncoded) {
+  // Only the path of a target sent to a proxy is encoded, so the brackets of
+  // an IPv6 host stay.
+  Server proxy;
+
+  std::string target;
+  proxy.set_pre_routing_handler([&](const Request &req, Response &res) {
+    target = req.target;
+    res.status = StatusCode::Forbidden_403;
+    return Server::HandlerResponse::Handled;
+  });
+
+  auto port = proxy.bind_to_any_port(HOST);
+  auto thread = std::thread([&]() { proxy.listen_after_bind(); });
+  auto se = detail::scope_exit([&] {
+    proxy.stop();
+    thread.join();
+    ASSERT_FALSE(proxy.is_running());
+  });
+
+  proxy.wait_until_ready();
+
+  {
+    Client cli("::1", 8080);
+    cli.set_proxy(HOST, port);
+
+    auto res = cli.Get("/a b");
+    ASSERT_TRUE(res) << "Error: " << to_string(res.error());
+    EXPECT_EQ("http://[::1]:8080/a%20b", target);
+  }
+
+#ifdef CPPHTTPLIB_SSL_ENABLED
+  {
+    SSLClient cli("::1", 8443);
+    cli.set_proxy(HOST, port);
+
+    auto res = cli.Get("/");
+    EXPECT_EQ(Error::ProxyConnection, res.error());
+    EXPECT_EQ("[::1]:8443", target);
+  }
+#endif
+}
+
 TEST(PathUrlEncodeTest, StreamingCRLFRejectedWhenPathEncodeDisabled) {
   // Nothing may reach the wire: a raw CR/LF target would split the request
   // line and inject headers.