Fix HttpURLConnection to not cache failures.

Found while investigating http://code.google.com/p/android/issues/detail?id=7787,
which turned out to be caused by unclear documentation. I've improved the
documentation somewhat, and stopped offering failed responses to the cache.

Bug: http://code.google.com/p/android/issues/detail?id=7787
Change-Id: Iec69769344af92345392c7ced817b0c823963524
6 files changed