Signal end-of-stream when remote sends a CLOSE notification

GlobusGSSCredentialImpl fails when the remote initiates a proper
SSL shutdown. The patch changes the logic such that rather
than throwing a generic error, the new ClosedGSSException is
thrown. The InputStream implementation recognizes this exception
and signals end-of-stream to the caller.
Merge pull request #136 from gbehrmann/fix/ssl-close

Signal end-of-stream when remote sends a CLOSE notification