Insert custom BinaryType values without the need of json::binary() ctor

[lemonpy]

Let's say that I have my basic_json type with my custom binary class, the following test fails

class Buffer : public std::vector<char>
{
...
};
using json = nlohmann::basic_json <
             std::map,
             std::vector,
             std::string,
             bool,
             std::int64_t,
             std::uint64_t,
             double,
             std::allocator,
             nlohmann::adl_serializer,
             Buffer /* binary_t */ >;

TEST_CASE("Use BinaryType assign ctor")
{
    json empty;
    Buffer buf(5, '1');
    empty["buf"] = buf;
    REQUIRE(empty["buf"].is_binary());
}

empty["buf"] results being an array.
Currently, you have to call json::binary() explicitly to insert a binary correctly no matter if you are inserting a BinaryType value into the json.