#1329 Improved spec compliance with etags

- RFC2616 requires that if an etag is given with the If-None-Match
  header, but doesn't match, then the If-Modified-Since header must be
  ignored.
- This was causing issues specifically when rolling back a deployment.
- Also moved the AssetsSpec out of the integrationtest project and into
  play-integration-test, to make it simpler.