The Main.WebRss should not a use a velocity variable named $request

The space parameter in the URL Main.WebRss allows the generation of a RSS feed for documents in a certain space.
Since the $request variable is overriden, the value for $request.space cannot be retrieved and the feed description will display null for the space value.
Steps to reproduce:

• go to http://localhost:8080/xwiki/bin/view/Main/WebRss?xpage=plain&outputSyntax=plain&space=Blog
• the feed description will be: RSS feed for document changes on space "null"