Allow conversion of string/text columns to array

Allow conversion of string/text columns to array so we can do:

```ruby
# column already exists as string
change_column :data_types, :string_1, :string, :array => true, :default => []
# or as text
change_column :data_types, :text_1, :text, :array => true, :default => []
```

LGTM given by: @nhance