core: fix for passing kwargs to constructor
core: fix for passing kwargs to constructor

* Fixes issue when passing kwargs to contructor and app is set to None.
  (closes #5)

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

inveniosoftware/invenio-db,inveniosoftware/invenio-db