/usr/include/boost/network/protocol/http/message/async_message.hpp:100:24: error: no match for 'operator=' (operand types are 'boost::optional<std::multimap<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, boost::network::impl::is_less_ignore_case_impl> >' and 'std::optional<std::multimap<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, boost::network::impl::is_less_ignore_case_impl> >')     retrieved_headers_ = make_optional(raw_headers);