Send correct headers for 304 responses
Send correct headers for 304 responses

The RFC specifies that the following headers should be sent with a 304
response, where the equivalent 200 response would include them:
Cache-Control, Content-Location, Date, ETag, Expires, and Vary.

See: http://tools.ietf.org/html/rfc7232#section-4.1

Fixes #85

evansd/whitenoise,evansd/whitenoise,evansd/whitenoise