json constructor does not support std::optional

[gzliudan]

I'm using clang 9.0 which support C++17, but below codes can't pass compile when I use nlohmann::json:

std::optional<int> value;
auto json = nlohmann::json{{"opt", value}};

instead, I have to use below codes:
if (value) {
    json = nlohmann::json{{"opt", *value}};
} else {
    json = nlohmann::json{{"opt", nullptr}};
}

[dota17]

Please See #1749 .
There is a PR #2117 , but it didn't merge into develop because there are some problems.

[nlohmann]

Duplicate of #1749.