Jetpack Search: Use full namespace for class_exists check for authenticated_requests to work

[rebeccahum]

Fixes #13243.

Changes proposed in this Pull Request:

• Per https://www.php.net/manual/en/function.class-exists.php:

If you are using aliasing to import namespaced classes, take care that class_exists will not work using the short, aliased class name - apparently whenever a class name is used as string, only the full-namespace version can be used

Therefore, class_exists( 'Client' ) will always return false. Instead, we should use the full namespace Automattic\Jetpack\Connection\Client.

Introduced in 3ed3de8 during re-factoring.

Since JP autoloads classes now, I think we might be able to just remove the check? Happy to re-commit the change if so.

Testing instructions:

1. Assuming JP Search module is enabled, add filter to enable authenticated requests via JP:

add_filter( 'jetpack_search_es_query_args', function( $es_query_args ) {
    $es_query_args['authenticated_request'] = true;
 
    return $es_query_args;
} );

2. Ensure es args have non-public content in search (e.g. drafts for post_type)
3. Perform search
4. Apply patch
5. Perform search again and see results

Proposed changelog entry for your changes:

• Bugfix: Authenticated requests to search will now display non-public content

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: September 3, 2019.
Scheduled code freeze: August 27, 2019

Generated by 🚫 dangerJS against 812881b

[mdawaffe]

Rebased to include #13245.

[mdawaffe]

Nice find - thanks!

[tyxla]

Thank you for this! 🙇