jch/opt_out — pull request #1 by chrishunt (2013-09-17 15:50:16+00:00)
Title: Fix bug in MemoryAdapter when already subscribed

`MemoryAdapter` will fail when subscribing to a list that is already subscribed.

Fixes:

```ruby
OptOut::MemoryAdapterTest#test_subscribe_when_already_subscribed:
NoMethodError: undefined method `delete' for nil:NilClass
```