data mapper API response cache is broken

[adieyal]

• most users were gettin 1970-01-01 etags therefore never updating
• we urgently needed to bust caches so we changed that to datetime.now()
• that might be bad if the client believes last-modified over the server response
• we need to understand why cache.get(key) returns None
• perhaps fix

Staging area (0.13 cont)

https://trello.com/c/dNjcdqsb/789-data-mapper-api-response-cache-is-broken