[Bug] Passing options to the request mutates "default options"

[nikophil]

There is a bug here because HttpOptions is not immutable, and HttpOptions::merge() mutates the default options, so this code will fail:

$browser = $this->browser()->setDefaultHttpOptions(['HTTP_SOME_HEADER' => 'xyz']);

$browser->get('/', ['HTTP_SOME_HEADER' => 'abc']);

$browser->get('/'); // 💥 uses "HTTP_SOME_HEADER' => 'abc'"

I propose that in 1.x we could clone the option:

    final public function request(string $method, string $url, $options = []): self
    {
        if ($this->defaultHttpOptions) {
-            $options = $this->defaultHttpOptions->merge($options);
+            $options = (clone $this->defaultHttpOptions)->merge($options);
        }

and I'd be in favor to make HttpOptions immutable in 2.x. This is a BC break, so this would be nice to do it now.

Also, we could make the BC break more explicit by adding the #[NoDiscard] attribute to all methods in HttpOptions, so at least in PHP 8.5 it would be very clear that this object is immutable.