Fix 'html5' theme support.
 * Require it to have a second argument when adding.
 * Merge, rather than replace, on second add.
 * Make current_theme_supports() work when two arguments are passed.

Adds unit tests.

props nathanrice for initial patch.
see #24932 for trunk.


git-svn-id: https://develop.svn.wordpress.org/trunk@25193 602fd350-edb4-49c9-b593-d223f7449a82