ADD - or `write(unless_exist:true)` -  is broken due to LocalStore

adding a breaking spec. The ideal flow is:

```ruby
@dalli = ActiveSupport::Cache.lookup_store(:dalli_store, 'localhost:11211')
@dalli.with_local_cache do
  @dalli.write('key', 123, unless_exist: true) # => true
  @dalli.write('key', 321, unless_exist: true) # => false
  @dalli.read('key') # => 123
end
```