On second thought, allow `posts_per_rss` to be set in `pre_get_posts` to override the `posts_per_rss` option. There are backwards compatibility concerns otherwise.

Props nacin.
Partially reverts [27455]. Updates the unit test. See #25380.



git-svn-id: https://develop.svn.wordpress.org/trunk@27456 602fd350-edb4-49c9-b593-d223f7449a82