oauth2server: fix support of SQLAlchemy-Utils
oauth2server: fix support of SQLAlchemy-Utils

* Addreses issue with new API of EncryptedType in
  SQLAlchemy-Utils>=0.28.2.  (closes #2629)

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

tiborsimko/invenio-oauth2server,inveniosoftware/invenio-oauth2server,tiborsimko/invenio-oauth2server,tiborsimko/invenio-oauth2server,inveniosoftware/invenio-oauth2server,inveniosoftware/invenio-oauth2server