Restore ResponseCache.get() / ResponseCache.put() methods on HttpResponseCache

This is required for interoperability between HttpResponseCache
with Java's HttpURLConnection and some scenarios on future Android
releases where an app has bundled an older version of OkHttp that uses
the ResponseCache API.
Merge pull request #656 from nfuller/ResponseCacheGetPut2

Restore ResponseCache.get() / ResponseCache.put() methods on HttpRespons...