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)
@christianv that's still the way the dates are being returned, it's the same SQL as in the dates on the Comments API.  

@nicolaasmatthijs I can add a spec in the controller spec to check for all the keys. I will do this and resubmit.