Allow posts_per_page option for pre_get_posts action hook on feed

[wokamoto]

This is a patch to fix an issue where posts_per_page option for pre_get_posts action hook does not work while is_feed() is true.

For example, this code can't change the number of posts per page in a feed.

<?php
function my_pre_get_posts_for_feed( $query ) {
  if ( is_admin() || ! $query->is_main_query() )
    return;

  if ( is_feed() ) {
    // Display 50 posts for the feed
    $query->set( 'posts_per_page', 50 );
  }
}
add_action( 'pre_get_posts', 'my_pre_get_posts_for_feed', 1 );

[nacin]

Thanks for this, wokamoto. This sounds familiar — there may be a similar bug report I couldn't find.

[wonderboymusic]

I am going to write a few tests for this.

[wonderboymusic]

In 27455:

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.

[wonderboymusic]

In 27456:

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.