Update how certain cache directives are handled

[JunTaoLuo]

• no-store on request:
  This should by-pass ShimResponseStream since we can't store any part of the response so there's no point in capturing the response
• no-cache on request:
  We can "validate" by asking the server to regenerate the response. As such we shouldn't skip the capturing of the response and update the cache entry if possible.
• max-stale
  The case where no limit is specified, which means any stale response can be served, is not handled.
• proxy-revalidate
  Same effect as must-revalidate

Also the header

• If-Unmodified-Since is being used incorrectly. The correct header to check for in when determining if content is modified is If-Modified-Since