fix: don't mutate the default http options when merging request options

[Amoifr]

Fixes #191.

HttpOptions::merge() mutates $this, so merging the per-request options into $this->defaultHttpOptions permanently rewrote the defaults:

$browser = $this->browser()->setDefaultHttpOptions(['headers' => ['x-foo' => 'bar']]);

$browser->get('/', ['headers' => ['x-foo' => 'baz']]);

$browser->get('/'); // still sends "x-foo: baz"

Cloning before merging, as suggested in the issue, keeps the defaults intact. HttpOptions only holds a single array, so a shallow clone fully isolates the state, and clone preserves the concrete class for HttpOptions subclasses.

The regression test lives in KernelBrowserTests, so it covers both the kernel and the HTTP browser; it fails on 1.x without the fix.

I left BrowserKitDriver::request() alone even though it merges into the caller's instance too: serverParameters is only ever populated through the Mink driver methods, never through the Browser API, so the merge is a no-op there. Happy to include it if you would rather have it defensive.

Making HttpOptions immutable (plus the #[NoDiscard] idea) is the 2.x half of the issue, so I kept it out of this one.

[kbond]

Thanks! I tacked on a couple of commits to cover the other options merge() leaks (query, json, ajax) and to give BrowserKitDriver the same treatment, thanks! 🎉