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)
@coyote : Note that ensuring that the property is returned as `date` is the most critical. The test would just be nice to avoid further regressions further down the line.