Invalid query parameters in `WP_Customize_Manager`

[dlh]

WP_Customize_Manager passes the number parameter to some WP_Query instances, but, as far as I can tell, WP_Query doesn't accept number.

For instance, in trunk running Twenty Seventeen, after this query: https://core.trac.wordpress.org/browser/trunk/src/wp-includes/class-wp-customize-manager.php#L1015

The value of $existing_posts_query->request is: "SELECT SQL_CALC_FOUND_ROWS wp_posts.ID FROM wp_posts WHERE 1=1 AND wp_posts.post_name IN [...] ORDER BY wp_posts.post_date DESC LIMIT 0, 10"

I assume the LIMIT wasn't supposed to be there, so the attached patch would replace number with posts_per_page.

[westonruter]

Tweak starter content unit test to ensure fix works

[westonruter]

@dlh wow, good catch! You're right.

PSA: get_pages() takes number, get_posts() has the alias numberposts, and WP_Query takes posts_per_page.

[westonruter]

In 39434:

Customize: Fix posts limit query arg for WP_Query from incorrect number to posts_per_page.

Props dlh.
Fixes #39022.

[westonruter]

In 39435:

Customize: Fix posts limit query arg for WP_Query from incorrect number to posts_per_page.

Merges [39434] onto 4.7 branch.
Props dlh.
Fixes #39022 for 4.7.