Add some unit tests for WP_HTTP::parse_url() to cover the <PHP 5.4.7 compatibility alterations.

These unit tests cover the expected vehaviour of certain combinations of URL's, but makes no attempt to test invalid URL structures, as PHP's behavious for invalid URL's is undefined (Some will be treated as paths, others fail, and it varies between PHP 5.4.7+ and <5.4.7).
This change also makes WP_HTTP::parse_url() protected in order to allow unit testing.
See #28001, #29886


git-svn-id: https://develop.svn.wordpress.org/trunk@29864 602fd350-edb4-49c9-b593-d223f7449a82