Ensure that a `has_children` parameter is given to `Walker::start_el()`.

Adds unit tests.

Props scribu, obenland.
Fixes #14041.

Built from https://develop.svn.wordpress.org/trunk@28824


git-svn-id: http://core.svn.wordpress.org/trunk@28628 1a063a9b-81f0-0310-95a4-ce76da25c4cd