base: wrapper doctest addition
base: wrapper doctest addition

* Adds doctests for `invenio.base.wrappers` that can be executed from
  source code folder: `python -m doctest -v invenio/base/wrappers.py`

Signed-off-by: Jiri Kuncar <2bdffcb416fed3002465740d19a053a39918cbe5@cern.ch>

tiborsimko/invenio-base,inveniosoftware/invenio-base,inveniosoftware/invenio-base,tiborsimko/invenio-base