fix download tests failing with hard-coded date

[sckott]

======================================================================
FAIL: test the addition of another predicate combiner
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Traceback (most recent call last):
  File "/Users/sckott/github/sac/pygbif/test/test-occurrences-download_request.py", line 43, in test_alternative_main_type
    'send_notification': 'true'})
AssertionError: {'creator': 'name', 'notification_address': ['em[94 chars] []}} != {'created': 2018, 'creator': 'name', 'notificati[94 chars]rue'}
- {'created': 2019,
?                ^

+ {'created': 2018,
?                ^

   'creator': 'name',
   'notification_address': ['email'],
   'predicate': {'predicates': [], 'type': 'or'},
   'send_notification': 'true'}

======================================================================
FAIL: test the creation of the predicate class
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Traceback (most recent call last):
  File "/Users/sckott/github/sac/pygbif/test/test-occurrences-download_request.py", line 31, in test_gbif_creation
    'send_notification': 'true'})
AssertionError: {'creator': 'name', 'notification_address': ['em[95 chars] []}} != {'created': 2018, 'creator': 'name', 'notificati[95 chars]rue'}
- {'created': 2019,
?                ^

+ {'created': 2018,
?                ^

   'creator': 'name',
   'notification_address': ['email'],
   'predicate': {'predicates': [], 'type': 'and'},
   'send_notification': 'true'}