0.12dev: Removed the XML processing instruction added in `render_template()` from [8618], as it resulted in a duplicated XML prolog for e.g. RSS feeds.
The default encoding of an XML document, in the absence of a BOM, is UTF-8, so it isn't strictly necessary to add an `encoding="UTF-8"` attribute. If we do want one, we should probably add it to the individual templates.
