Respect the `fields` arg when passed to `get_children()`.

Fixes #22208.


Built from https://develop.svn.wordpress.org/trunk@25160


git-svn-id: http://core.svn.wordpress.org/trunk@25139 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Support `id=>name` and `id=>slug` values for `fields` arg in `get_terms()`. Adds unit tests.

Props mikeschinkel.

Fixes #13661.


Built from https://develop.svn.wordpress.org/trunk@25161


git-svn-id: http://core.svn.wordpress.org/trunk@25140 1a063a9b-81f0-0310-95a4-ce76da25c4cd