general: rely on MutableMapping for SmartDict

[hachreak]

• [ ] Rely on MutableMapping for SmartDict and don't override the following methods: __contains__, keys, items, values, get, __eq__, __ne__, pop, popitem, clear, update, and setdefault.
• https://docs.python.org/2/library/collections.html#collections-abstract-base-classes
• https://github.com/inveniosoftware/invenio/blob/c4d74fd3945567d43888b8e0e19154b27ea445a9/invenio/utils/datastructures.py
• [ ] Move tests for SmartDict from Invenio to JSONAlchemy

See https://github.com/inveniosoftware/invenio/issues/2029