Fix the extras_keys/extras_items methods of Node

[giovannipizzi]

This line
https://github.com/aiidateam/aiida_core/blob/16f105ecd820eb04358ac67c4c954cd0db93ad36/aiida/orm/nodes/node.py#L623
and this line
https://github.com/aiidateam/aiida_core/blob/16f105ecd820eb04358ac67c4c954cd0db93ad36/aiida/orm/nodes/node.py#L615
are wrong.
First, extras is a property and should not be called.
In any case, we shouldn't go via extras but via the methods to get only the keys, for instance. See, as a reference, the implementation for attributes:
https://github.com/aiidateam/aiida_core/blob/16f105ecd820eb04358ac67c4c954cd0db93ad36/aiida/orm/nodes/node.py#L496

This also means this is not tested, so tests should be added for these methods (and maybe we might want to check on coverage.io/coveralls which other methods are currently untested).