fix ambiguous for HandlerWithResponse and Handler in set_error_handler

using lambda expression as Handler in set_error_handler will cause ambiguous.
Template forwarding can forward HandlerWithResponse to the correct overloading function

Author: HerrCai0907

httplib.h

@@ -231,6 +231,7 @@ using socket_t = int;
 #include <string>
 #include <sys/stat.h>
 #include <thread>
+#include <type_traits>
 #include <unordered_map>
 #include <unordered_set>
 #include <utility>
@@ -871,8 +872,13 @@ class Server {
   Server &set_default_file_mimetype(const std::string &mime);
   Server &set_file_request_handler(Handler handler);
 
-  Server &set_error_handler(HandlerWithResponse handler);
-  Server &set_error_handler(Handler handler);
+  template <class ErrorHandlerFunc>
+  Server &set_error_handler(ErrorHandlerFunc &&handler) {
+    return set_error_handler_impl(
+        std::forward<ErrorHandlerFunc>(handler),
+        std::is_convertible<ErrorHandlerFunc, HandlerWithResponse>{});
+  }
+
   Server &set_exception_handler(ExceptionHandler handler);
   Server &set_pre_routing_handler(HandlerWithResponse handler);
   Server &set_post_routing_handler(Handler handler);
@@ -934,6 +940,9 @@ class Server {
   size_t payload_max_length_ = CPPHTTPLIB_PAYLOAD_MAX_LENGTH;
 
 private:
+  Server &set_error_handler_impl(HandlerWithResponse handler, std::true_type);
+  Server &set_error_handler_impl(Handler handler, std::false_type);
+
   using Handlers =
       std::vector<std::pair<std::unique_ptr<detail::MatcherBase>, Handler>>;
   using HandlersForContentReader =
@@ -5797,12 +5806,14 @@ inline Server &Server::set_file_request_handler(Handler handler) {
   return *this;
 }
 
-inline Server &Server::set_error_handler(HandlerWithResponse handler) {
+inline Server &Server::set_error_handler_impl(HandlerWithResponse handler,
+                                              std::true_type) {
   error_handler_ = std::move(handler);
   return *this;
 }
 
-inline Server &Server::set_error_handler(Handler handler) {
+inline Server &Server::set_error_handler_impl(Handler handler,
+                                              std::false_type) {
   error_handler_ = [handler](const Request &req, Response &res) {
     handler(req, res);
     return HandlerResponse::Handled;