[13.x] Add withoutHeader() and withoutHeaders() to HTTP client Pendin…

[Amirhf1]

This PR adds withoutHeader() and withoutHeaders() methods to the HTTP client's PendingRequest, allowing removal of headers before a request is sent.

The client currently supports adding headers (withHeader(), withHeaders(), replaceHeaders()) but offers no fluent way to remove one. This mirrors the withoutHeader() method recently added to ResponseTrait and the existing withCookie()/withoutCookie() symmetry, completing the header API.

Use Cases

• Removing a default header set by a base client or Http::macro
• Stripping a header for a single request without rebuilding the client
• Removing sensitive headers (e.g. X-Debug, Authorization) before proxying

Usage

Http::withHeaders(['X-Debug' => '1'])
    ->withoutHeader('X-Debug')
    ->withoutHeaders(['X-Trace', 'X-Internal'])
    ->get('https://example.com');