When setting `posts_per_page` in `WP_Query::get_posts()`, check for `is_feed()` in the same place where `posts_per_page` is set when it is not already set. Previously, when `is_feed()` was true, `posts_per_page` would be arbitrarily overwritten. This fix allows `posts_per_page` to be set during `pre_get_posts` when `is_feed()` is true and not be overwritten.

Adds unit test.

Props wokamoto.
Fixes #25380.



git-svn-id: https://develop.svn.wordpress.org/trunk@27455 602fd350-edb4-49c9-b593-d223f7449a82