[7.x] Avoid using array_merge_recursive in HTTP client

[themsaid]

Using array_merge_recursive can lead to some configuration methods to set values to an array if called multiple times.

For example:

$client = Http::withToken('Token-A')->post();

// Do something which requires the original token

$client->withToken('Token-B')->post();

// Do something else that requires a different token.

Calling withToken multiple times will lead to having the token header value as an array.