CYB-375 The date is no longer returned in the /gallery/index feed

 * adding the 3 date fields that are taken from the Canvas API return values
   for each asset type to the selected values returned

[CYB-375 The date is no longer returned in the /gallery/index feed](https://jira.ets.berkeley.edu/jira/browse/CYB-375)
The dates are coming back as an `int8` property instead of a `date` property.
Would it be possible to add an automated test that checks the output of the /gallery/index feed and verifies that all of the required properties are present?