Fix small bug with dirty attributes and list assignment as an array.

I notice that when you are not using the default delimiter and make the assignment with an array, the attributes are marked as dirty when they shouldn't. eg:

```ruby

ActsAsTaggableOn.delimiter = ';'
a.topic_list 
=>
['topic1', 'topic2'] 
a.topic_list = ['topic1', 'topic2'] 
a.changes
=>
{"topic_list"=>["Politics; Test", ["Politics", "Test"]]} 

```

This commit attempts to fix this problem.

Cheers!