default and custom error pages response

+ refactoring in response.cpp (functions split)
+ added Client::clear()
+ added replace_all_substr() in utils.cpp

srcs/utils.hpp

@@ -32,5 +32,6 @@ std::string					itos(int n);
 std::string					trim(std::string str, char c);
 http_method					str_to_http_method(std::string &str);
 std::string					http_methods_to_str(unsigned int methods);
+void						replace_all_substr(std::string &str, const std::string &ori_substr, const std::string &new_substr);
 
 #endif