The array-compatibility object we started returning in r37428 unfortunately isn't enough like an array. In particular, `is_array()` checks fail, despite the object implementing ArrayAccess. Mea culpa.
This moves the WP_HTTP_Response object to a new http_response key in the array, and changes the value back to an actual array.
Fixes#37097.
See #33055.
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Also use 'back-compat' in some inline comments where backward compatibility is the subject and shorthand feels more natural.
Note: 'backwards compatibility/compatibile' can also be considered correct, though it's primary seen in regular use in British English.
Props ocean90.
Fixes#36835.
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