Boone Gorges 5c0ddb8602 Default $parent in category_exists() should default to null rather than 0.
[29863] made the corresponding change in `term_exists()`. Failure to change the
default value in `category_exists()` meant that an unspecified value for
`$parent` would limit results to top-level categories.

Includes unit tests and corrected function documentation.

Props hissy.
Fixes #30975 for trunk.
Built from https://develop.svn.wordpress.org/trunk@31140


git-svn-id: http://core.svn.wordpress.org/trunk@31121 1a063a9b-81f0-0310-95a4-ce76da25c4cd
2015-01-11 01:26:27 +00:00
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