Array to string conversion warning after r50157

[dcavins]

When using the new REST API feature that allows one to add the include_children parameter to a taxonomy-term-filtered request, like

const query = {
    categories: {
        terms: [ 3, 5, 7 ],
        include_children: true,
    },
};

(example taken from ​https://make.wordpress.org/core/2021/02/23/rest-api-changes-in-wordpress-5-7/)

I'm encountering the warning:
PHP Warning: Array to string conversion in /Users/cavinsd/Local Sites/caresnet/app/public/wp-includes/class-wp.php on line 361

The error refers to this line:

$this->query_vars[ $t->query_var ] = str_replace( ' ', '+', $this->query_vars[ $t->query_var ] );

which is expecting a flat taxonomy parameter, not a multidimensional array.

Thanks for adding the include_children parameter; it makes working with the REST API much simpler in some cases!

[dcavins]

Change cleanup logic depending on whether query variable is a string or an array.

[antonvlasenko]

When I use a query like the one in the example, I cannot replicate the issue, at least not with a standard WordPress setup. Here are the queries I've tested:

1. wp/v2/posts?categories%5Bterms%5D%5B0%5D=1&categories%5Bterms%5D%5B1%5D=2&categories%5Bterms%5D%5B2%5D=4&categories%5Binclude_children%5D=1
2. wp/v2/posts?categories%5Bterms%5D%5B0%5D=3&categories%5Bterms%5D%5B1%5D=5&categories%5Bterms%5D%5B2%5D=7&categories%5Binclude_children%5D=true

I am curious to know the exact query used to reproduce the issue.