Add SSLSession accessors to HttpsURLConnection and SecureCacheResponse

It would be useful to know the TLS protocol version negotiated between a
client and server. However, the HttpsURLConnection class does not provide a
means to obtain this information (even though it is available, buried deep in
in the sun.security.ssl.SSLSessionImpl class object inside the previously
mentioned public class).

The existing getCipherSuite() method on HttpsURLConnection is not sufficient,
as many of the cipher suites are supported across multiple TLS versions.
SSLSocket does provide access to the SSLSession object that can provide this
information, but the SSLSocket itself is not always exposed (such as when
used within HttpsURLConnection),

So, please expand the API of classes that wrap around SSLSocket to allow a
pass-through to SSLSession.getProtocol().