Fix ResponseCacheTest for Android

Pull #1011 broke ResponseCacheTest on Android.

The test was asserting a header generated
using a copy of the old broken implementation.

The test now just uses HttpDate, which now has its own
tests.
Bump Okio dependency.
Merge pull request #1020 from square/jw/okio-update

Bump Okio dependency.
Merge pull request #1018 from nfuller/FixResponseCacheTestOnAndroid

Fix ResponseCacheTest for Android