Update serializers.js

This fixes the issue you run into when trying to use a TimeUUID for a column name on a columnFamily.insert(). The column name cannot be a TimeUUID object, but needs to be a String. (Object keys cannot be objects themselves.)

Example:
```javascript
keyspace.get('columFamily', function(err, cf) {
    var timeUUID = TimeUUID.fromTimestamp(new Date());
    var column = {};
    column[timeUUID] = JSON.stringify({oy: 'yo'});

    cf.insert('key', column, function(err) {
      if (err) throw(err);
    });
 })
```