Improve 'alias_of' handling in wp_insert_term() and wp_update_term().

Using get_term_by() rather than direct SQL queries to fetch the alias term
fixes a number of issues:

* Object cache for aliased term is properly cleared after update.
* If the aliased term is in the object cache, it's served from there, saving a database query.
* Duplicate 'edit_terms' and 'edited_terms' hooks can be removed.
* Fix a PHP notice when the 'alias_of' term is not found.
* Prevent the incorrect creation of a new term group for the primary term when the 'alias_of' term is not found.

Adds unit tests for 'alias_of' functionality in both functions.

Fixes #29848.

git-svn-id: https://develop.svn.wordpress.org/trunk@29862 602fd350-edb4-49c9-b593-d223f7449a82
Fix term_exists() for parent = 0.

Passing a 0 (or '0') as the 'parent' param of term_exists() should limit
results to terms with no parent.

Adds unit test.

Fixes #29851.

git-svn-id: https://develop.svn.wordpress.org/trunk@29863 602fd350-edb4-49c9-b593-d223f7449a82
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
Don't bail out of term_exists() when term sanitizes to an empty string.

This change brings return values for failures of this sort in line with other
failed lookups in term_exists(): a null value is now returned in all cases
where the queried term is not found.

Adds unit test for the sanitization issue. Modifies existing unit test to
reflect the change in return value for empty term strings.

Props boonebgorges, georgestephanis.
Fixes #29589.

git-svn-id: https://develop.svn.wordpress.org/trunk@29865 602fd350-edb4-49c9-b593-d223f7449a82