This PR fixes an issue where subclass methods of Headers were not being called correctly.

Problem

When a class extends Headers, the subclass's methods (like append) were not accessible or called properly. This was caused by the Proxy instance in the Headers constructor, which intercepted method calls and directed them to the URLSearchParams prototype, bypassing any overrides defined in subclasses.

Solution

• Removed the Proxy from the constructor.
• Re-implemented the proxied methods in the Headers class to maintain the expected behavior and allowing overrides by subclasses.

Example

import SuperHeaders from "@mjackson/headers"; // Extends `Headers` in Remix

// Correctly initialized. The constructor calls a setter directly from `SuperHeaders.prototype`.
console.log(new SuperHeaders({ cookie: "foo=123" }).cookie.size); // 1

// Incorrectly initialized. The constructor calls `this.append`, which is unexpectedly not handled
// by the subclass due to the Proxy.
console.log(new SuperHeaders([["cookie", "foo=123"]]).cookie.size); // 0

cc: @mjackson