fix as_8bit_ascii changes argument's encoding

I met encoding problem after save.

Example in `rails console`:
```
>> post.tag_list = ["\u3042"]
=> ["あ"]
>> post.tag_list[0].encoding
=> #<Encoding:UTF-8>
>> post.save
(snip queries)
=> true
>> post.tag_list[0].encoding
=> #<Encoding:ASCII-8BIT>
```

And logger says `log writing failed. "\xE3" from ASCII-8BIT to UTF-8` when running specs.