Coding Standards: Move wp-includes/class-http.php to wp-includes/class-wp-http.php.

This renames the file containing the `WP_Http` class to conform to the coding standards.

This commit also includes:

- A new `class-http.php` that includes the new file, for anyone that may've been including the file directly.
- Replaces references to the old filename with the new filename.

Follow-up to [8516], [13274], [33748].

Fixes #54389. See #53359.

git-svn-id: https://develop.svn.wordpress.org/trunk@52026 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov