message: Fix unboxing string and binary values in CppContext
diff: @@ -146,7 +146,7 @@ inline StringData CppContext::unbox(util::Any& v, bool, bool) const
  {
  if (!v.has_value())
  return StringData();
  - auto value = any_cast<std::string&>(v);
  + auto& value = any_cast<std::string&>(v);
  return StringData(value.c_str(), value.size());
  }
  @@ -155,7 +155,7 @@ inline BinaryData CppContext::unbox(util::Any& v, bool, bool) const
  {
  if (!v.has_value())
  return BinaryData();
  - auto value = any_cast<std::string&>(v);
  + auto& value = any_cast<std::string&>(v);
  return BinaryData(value.c_str(), value.size());
  }
  