Change the cache to have non-shared semantics.

This means we'll cache responses that use an 'Authorization' header. This
means OkHttp's cache shouldn't be used on middleboxes that sit between
user agents and the origin server; in practice this is never a use case
for OkHttp.

Fixes https://github.com/square/okhttp/issues/1035
Merge pull request #1113 from square/jwilson_1101_not_shared

Change the cache to have non-shared semantics.