support mongoc 2.x

[baidwwy]

https://github.com/mongodb/mongo-c-driver/blob/632143bf35a309052481f16afe3ee4260360ad05/src/libbson/NEWS#L136-L139

lua-mongo/src/bson.c

Lines 64 to 70 in 74f15a0

	static int m__tostring(lua_State *L) {
	size_t len;
	char *str = bson_as_json(checkBSON(L, 1), &len);
	lua_pushlstring(L, str, len);
	bson_free(str);
	return 1;
	}

static int m__tostring(lua_State *L) {
	size_t len;
#if BSON_MAJOR_VERSION == 1
	char* str = bson_as_json(checkBSON(L, 1), &len);
#elif BSON_MAJOR_VERSION == 2
	char *str = bson_as_legacy_extended_json(checkBSON(L, 1), &len);
#endif
	lua_pushlstring(L, str, len);
	bson_free(str);
	return 1;
}

[neoxic]

Thanks for the heads-up! I'll take a look into it.