issue_28 tests added to catch invalid object types for helpers

Author: nickmaccarthy

tests.py

@@ -6,7 +6,6 @@
 import arrow
 from datetime import datetime as pydatetime
 from datetime import timedelta
-from datetime import timezone as pydttz
 from datemath import dm, datemath
 from datemath.helpers import DateMathException as DateMathException
 from dateutil import tz