Can reset error handler with default handler

In spdlog::logger::set_error_handler, adds the possibility to reset the
handler with the default one, by giving a null or empty handler as
argument.

include/spdlog/details/logger_impl.h

@@ -250,6 +250,14 @@ inline void spdlog::logger::set_level(spdlog::level::level_enum log_level)
 
 inline void spdlog::logger::set_error_handler(spdlog::log_err_handler err_handler)
 {
+    if (!err_handler)
+    {
+        _err_handler = [this](const std::string &msg)
+        {
+            this->_default_err_handler(msg);
+        };
+    }
+    else    
         _err_handler = err_handler;
 }