Always sanitize user_nicename in wp_insert_user().

Previously, a 'user_nicename' parameter passed into the function was
unsanitized. This could result in a mismatch between the sanitized nicename
generated automatically at user creation, resulting in broken author archive
permalinks.

Props joemcgill.

Fixes #29696.
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git-svn-id: http://core.svn.wordpress.org/trunk@29585 1a063a9b-81f0-0310-95a4-ce76da25c4cd