corrected uppercase DatePreset to lowercase for AdSet

[zachliu]

trafficstars

The DatePreset values are lower cases. Simple fix.

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[zachliu]

Because of these uppercase values, we should expect a very misleading warning message

facebook_business/utils/api_utils.py:30: UserWarning: value of date_preset might not be compatible.  Expect date_preset_enum;  got <class 'str'>
  warnings.warn(message)

if we do something like

campaign.get_ad_sets(params={'date_preset': 'last_30d'})

because last_30d doesn't match with LAST_30D :crying_cat_face:

And if you are using the STRICT_MODE, your code would fail with the same confusing message.