Added that render :json will automatically call .to_json unless its being passed a string [DHH] Added Mime::Type.register_alias for dealing with different formats using the same mime type [DHH]

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@7520 5ecf4fe2-1ee6-0310-87b1-e25e094e27de