broken from_json implementation for pair and tuple

[theodelrieu]

I just realized that I introduced a bug in the develop branch.

The tests pass, because I forgot to add a test that retrieves a pair and a tuple...
That's really shameful, but luckily we haven't released yet.

// no need for variadic here by the way
template<typename BasicJsonType, typename... Args>
void from_json(const BasicJsonType& j, std::pair<Args...>& p)
{
    // broken, must use at().get<>()
    p = {j.at(0), j.at(1)};
}

template<typename BasicJsonType, typename Tuple, std::size_t... Idx>
void from_json_tuple_impl(const BasicJsonType& j, Tuple& t, index_sequence<Idx...>)
{
    // same thing here
    t = std::make_tuple(j.at(Idx)...);
}

[nlohmann]

Can you open a PR?

[theodelrieu]

Should not take really long, I'm on it.