Fix alert regression from #95
Fix alert regression from #95

A regression from #95 led to all alerts being sent as unencrypted. This
meant, once a handshake was completed, but an unencrypted alert was
sent, the server failed to parse the alert and the close would not take
place. Previously the Close functionality was untested so it managed to
slip through. I've updated the E2E tests here to exercise the close
functionality on both the client and the server. The E2E tests now
properly fail with the regression.

pion/dtls