causes/mock_redis — pull request #75 by ches (2014-01-22 08:59:27+00:00)
Title: Raise on empty array given to `del`

Following from #74, DEL should get the same treatment:

MockRedis:

```ruby
$mock_redis.del [] # => 0
```

redis-rb:

```ruby
$redis.del [] # => Redis::CommandError
```