Ensure that `shortcode_unautop()` treats `&nbsp;` like whitespace.
`shortcode_unautop()` and `wptexturize()` now use `wp_spaces_regexp()` instead of raw regex.

Adds unit tests.

Props miqrogroove.
See #27588.


git-svn-id: https://develop.svn.wordpress.org/trunk@28716 602fd350-edb4-49c9-b593-d223f7449a82