Admin Menu: Use `.dashicons-before` as default $img_class.

props kovshenin for noticing and testing.
see #26630.
Built from https://develop.svn.wordpress.org/trunk@27425


git-svn-id: http://core.svn.wordpress.org/trunk@27272 1a063a9b-81f0-0310-95a4-ce76da25c4cd