RSS and Atom feeds do not contain an XML header

An XML parser that is expecting a well formed document will not parse Hudson's
RSS or Atom feeds because they do not start with something like:

<?xml version="1.0" encoding="UTF-8"?>

For example, this breaks Rome.

The fix would be to add this line to core/src/main/resources/hudson/atom.jelly
and rss20.jelly in the appropriate place.