message: fix compile errors on Mac OS
diff: @@ -107,10 +107,10 @@ int http_parser::on_url(const char* s, size_t n) { return 0; }
  int http_parser::on_header_field(const char* s, size_t n)
  {
  if (m_state == eParsingField) {
  - m_current_field += {s, n};
  + m_current_field += std::string(s, n);
  } else {
  store_current_header_field();
  - m_current_field = {s, n};
  + m_current_field = std::string(s, n);
  m_state = eParsingField;
  }
  @@ -121,10 +121,10 @@ int http_parser::on_header_field(const char* s, size_t n)
  int http_parser::on_header_value(const char* s, size_t n)
  {
  if (m_state == eParsingField) {
  - m_current_value = {s, n};
  + m_current_value = std::string(s, n);
  m_state = eParsingValue;
  } else
  - m_current_value += {s, n};
  + m_current_value += std::string(s, n);
  return 0;
  @@ -151,7 +151,7 @@ int http_parser::on_headers_complete()
  int http_parser::on_status(const char* s, size_t n)
  {
  - m_http_status += {s, n};
  + m_http_status += std::string(s, n);
  return 0;
  }
  