When rich editor is enabled, post's content is not properly escaped

[xknown]

If an user has unfiltered_html capability and rich editor is enabled on his profile, post's content is not properly escaped.

Steps to reproduce the problem:

1. Write a new post/page using the "code" view with the following content </textarea><script>alert(123)</script>
2. Press "Save and Continue editing" button.

It seems the problem is on ​line 691 of wp-includes/post.php.

[ryan]

Fixed for #6449