fix the bug

[dota17]

This is a new PR for #2181.
From CI logs, the test SECTION("PR #2181 - regression bug with lvalue") has no error for this new PR.
I just added back the original method and then there would be two methods:

ValueType value(const typename object_t::key_type& key, const ValueType& default_value) const //  added back
...
ValueType value(const typename object_t::key_type& key, ValueType && default_value) const

But to be cautious, maybe some more testcases should be added.

[coveralls]

Coverage remained the same at 100.0% when pulling 30dd9bd on dota17:fix_2181 into 4c7bd01 on nlohmann:develop.

[nlohmann]

Please also add a test to cover the added function for JSON pointers.