API: Use HTMLText as default for RSS Description

To allow for custom RSS descriptions (i.e. provided by a method rather than a DB field) and avoid double escaping a better default class for Description is 'HTMLText'. http://www.silverstripe.org/customising-the-cms/show/11606#post319519
Merge pull request #1137 from jonom/patch-2

API: Use HTMLText as default for RSS Description