Use correct connection type in shared deck download (http/https).

Recently, AnkiWeb changed its download link for shared decks to a non-HTTPS connection. This was causing AnkiDroid to crash as it failed to typecast the HTTP connection to an HTTPS one.

[Issue 1754](https://code.google.com/p/ankidroid/issues/detail?id=1754)
[Discussion here](https://anki.tenderapp.com/discussions/ankiweb/87-shared-deck-downloads-are-not-done-over-https).