Return HttpResponseCache to previous (e.g. KitKat) behavior

This change required to handle this Android scenario:

When this version of OkHttp is packaged into Android in a future
release (in com.android.okhttp...) there is a possibility that
an older version of OkHttp bundled with application
(in com.squareup.okhttp...) will pick up the newer HttpResponseCache
via ResponseCache.getDefault(). It will attempt to use the get() / put()
methods. Commit e2599be9e073f1be8e929597c358e2664ab0c9a9 was
intended to restore HttpResponseCache get() and put() and avoid the
UnsupportedOperationException that would be thrown.

Unfortunately, it didn't quite do it:

In KitKat and older versions OkHttp, in HttpResponseCache the behavior
was to ignore calls to put(URI, URLConnection) including
any HttpURLConnection/HttpsURLConnection that wasn't its own
(i.e. only ones from com.android.okhttp would be cached). The previous
fix to HttpResponseCache re-introduced get()/put() but didn't implement
the necessary instanceof checks.

Jesse also informs me that coexistance with other implementations
of HttpURLConnection is not required (since OkHttp replaces the
HTTP implementation on the desktop). I have also since discovered
that HttpResponseCache didn't implement the cache-control logic
required to follow the letter of the Java API anyway, which means
that HttpResponseCache is not suitable for use in cases where
the Http(s)URLConnection implementation expects a fully compliant
implementation.

Also addressed nits from e2599be9e073f1be8e929597c358e2664ab0c9a9.