In `WP_HTTP_IXR_Client`:
* declare `$scheme` as a property, it doesn't exist on the parent class
* declare `$error` as a property of type `IXR_Error`, instead of it being inferred as `boolean` from the parent class

See #30224.


git-svn-id: https://develop.svn.wordpress.org/trunk@30179 602fd350-edb4-49c9-b593-d223f7449a82