Unserialize `get_metadata()` results when 'key' is omitted.

Props mattkeys, nacin.
Fixes #15030.
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Twenty Fifteen: Change version compare string to '4.1-alpha' to avoid an error for alpha and beta testers during development.

fixes #30137.
Built from https://develop.svn.wordpress.org/trunk@30116


git-svn-id: http://core.svn.wordpress.org/trunk@30116 1a063a9b-81f0-0310-95a4-ce76da25c4cd