Clearing a pre-existing value in a Set

To repro:

1. Create a table with one of its columns being a set.
2. Through the plugin, create a new instance and assign the set some value, then save.
3. Now, try to set the set value to something completely new (e.g., don't just append) and re-save it.

You should get an error like "CassandraCQL::Error::InvalidRequestException: Invalid list literal for labels of type set<text>". It looks like the problem is that the UPDATE command is issued with square brackets instead of curly. For example, it tries 
```
UPDATE google_mail_message_label_history_data SET labels = [] WHERE service_id = 234 AND gm_msg_id = 'abc'
```
 when CQL wants 
```
UPDATE google_mail_message_label_history_data SET labels = {} WHERE service_id = 234 AND gm_msg_id = 'abc'
```

A bit of a sample repro:
---------
```
datum = GoogleMailMessageLabelHistoryDatum.new(:service_id => service.id, :gm_msg_id => gm_msgid)
datum.labels = ["label1"]
datum.save!

datum.labels = ["clearit"]
datum.save!
```

As a sidenote, this does work if on the second declaration you declare the new label as a Set, 
```
datum.labels = Set['clearit']
```