fix: support objects with null prototype as HeadersInit (#81)

Currently, if an object is created with the null prototype, e.g.

```typescript
const headers = Object.create(null);
```

attempting to instantiate a `Headers` object with this polyfill throws
an error:

```
TypeError: Cannot read properties of undefined (reading 'name')
```

because `headers.constructor` is undefined.

@kettanaito

---------

Co-authored-by: Artem Zakharchenko <kettanaito@gmail.com>

Author: tmisirpash

src/Headers.test.ts

@@ -40,6 +40,13 @@ describe('constructor()', () => {
     expect(headers.get('accept')).toEqual('*/*')
   })
 
+  it('can be created given an object with a null prototype', () => {
+    const headersInit = Object.create(null)
+    headersInit.accept = '*/*'
+    const headers = new Headers(headersInit)
+    expect(headers.get('accept')).toEqual('*/*')
+  })
+
   it('duplicates values for the same header names with different casing', () => {
     const headers = new Headers({
       'accept-encoding': 'gzip, deflate, br',

src/Headers.ts

@@ -25,7 +25,7 @@ export class Headers {
      * `Headers` because that class may not be defined in Node or jsdom.
      */
     if (
-      ['Headers', 'HeadersPolyfill'].includes(init?.constructor.name) ||
+      ['Headers', 'HeadersPolyfill'].includes(init?.constructor?.name) ||
       init instanceof Headers ||
       (typeof globalThis.Headers !== 'undefined' &&
         init instanceof globalThis.Headers)