Default value for $credentials parameter in wp_signon() should be an array, not a string.

props tivnet.
fixes #27130.
Built from https://develop.svn.wordpress.org/trunk@27350


git-svn-id: http://core.svn.wordpress.org/trunk@27201 1a063a9b-81f0-0310-95a4-ce76da25c4cd