Vangelis Banos 56f0118374 Replace timestamp parameter with more generic request/response syntax
Replace timestamp parameter with more generic extra_response_headers={}

When request has --header ``Warcprox-Meta: {\"accept\":[\"capture-metadata\"]}"``
Response has the following header:
``Warcprox-Meta: {"capture-metadata":{"timestamp":"2017-10-31T10:47:50Z"}}``

Update unit test
2017-10-31 10:49:10 +00:00
..
2016-04-06 19:37:55 -07:00
2017-10-19 22:22:14 +00:00