Don't include the default columns="3" in a gallery shortcode.

Shortcode attributes are strings; fix a variable type error in wp.media.gallery defaults.

Merges [23247] to the 3.5 branch.

props adamsilverstein.
fixes #22919.
HTTP API: Use win_is_writable() on Windows when checking the directory to which we are streaming.

props dd32.
see #22900.
for the 3.5 branch only.
When copying new bundled themes & plugins, bail early if the distro doesn't include the bundled item. This prevents us from creating an empty directory in the destination when the source doesn't exist. See #22856. Merges [23179] to the 3.5 branch
Upgrade: When upgrading from WordPress 3.5, if an empty twentytwelve theme directory exists, remove it and install Twenty Twelve. See #22856. For the 3.5 branch