tests: regression on complex dictionaries

* Adds tests to check regression on complex dictionaries.  (closes #4)

Co-authored-by: Erik Wickstrom <erik@erikwickstrom.com>
Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
tests: check basestring handling

* Checks issue when basestring was handled as iterable. (addresses #6)

Signed-off-by: Jiri Kuncar <jiri.kuncar@gmail.com>