SqlBypass session store doesn't use ActiveRecord::Base connection

All the methods in ActiveRecord::SessionStore::SqlBypass use
@@connection class var, instead of the homonym class method. This of
course doesn't not guarantee that var isn't nil.

Try it by yourself:
# config/initializers/session_store.rb
ActionController::Base.session_store = :active_record_store
ActiveRecord::SessionStore.session_class =
ActiveRecord::SessionStore::SqlBypass

Then try to start the server and use sessions, it should raise a
`NoMethodError`.

I created a patch for solving this issue, but not the whole problem.
Since there aren't unit tests for `ActiveRecord` session stores, but
only functional tests in `ActionPack`.

As you can see from the patch I replaced `@@connection` with
`connection` only where covered by tests.

LH Ticket: http://bit.ly/FGiaD

Cheers,
Luca