Fix HttpResponseCacheTest in anticipation of an OkHttp upgrade.

OkHttp recently changed the behavior of their caching with
commit e74e3f3bf744ef7f4d8ee724a7cf2347e486cfab - it is now
neccessary to close the inputstream (or disconnect the
HttpURLConnection) for a response to be cached.

This change is (effectively) a no-op prior to the upgrade.

The behavior is undefined as to whether closing the
input stream is required for caching. OkHttp's new behavior
is consistent with other HttpURLConnection implementations
tried.

Change-Id: Iaf57371651296ac84850971ef60a9338cead57c0