Allow XML attributes with colons to be read by kses.

The attribute would still need to be whitelisted to get through the filters.

props jorbin.
fixes #17847.

Built from https://develop.svn.wordpress.org/trunk@27707


git-svn-id: http://core.svn.wordpress.org/trunk@27546 1a063a9b-81f0-0310-95a4-ce76da25c4cd