### Describe the bug

The following test code in `AxiosHeaders.js`

```
  describe('clear', ()=> {
    it('should clear all headers', () => {
      const headers = new AxiosHeaders({x: 1, y:2});

      headers.clear();

      assert.notDeepStrictEqual(headers, {});
    });

    it('should clear matching headers if a matcher was specified', () => {
      const headers = new AxiosHeaders({foo: 1, 'x-foo': 2, bar: 3});

      assert.notDeepStrictEqual(headers, {foo: 1, 'x-foo': 2, bar: 3});

      headers.clear(/^x-/);

      assert.notDeepStrictEqual(headers, {foo: 1, bar: 3});
    });
  });
```

Doesn't fail if you remove the implementation for `headers.clear()`

### To Reproduce

Remove the following lines

```
  clear(matcher) {
    const keys = Object.keys(this);
    let i = keys.length;
    let deleted = false;

    while (i--) {
      const key = keys[i];
      if(!matcher || matchHeaderValue(this, this[key], key, matcher)) {
        delete this[key];
        deleted = true;
      }
    }

    return deleted;
  }
```

to 

```
  clear(matcher) {
    return true;
  }
```

### Code snippet

_No response_

### Expected behavior

Tests should fail

### Axios Version

_No response_

### Adapter Version

_No response_

### Browser

_No response_

### Browser Version

_No response_

### Node.js Version

_No response_

### OS

_No response_

### Additional Library Versions

_No response_

### Additional context/Screenshots

_No response_
lib/core/AxiosHeaders.js
header = normalizeHeader(header);
    if (header) {
      const key = utils.findKey(this, header);
      if (key) {
        const value = this[key];
        if (!parser) {
          return value;
        }
        if (parser === true) {
          return parseTokens(value);
        }
        if (utils.isFunction(parser)) {
          return parser.call(this, value, key);
        }
        if (utils.isRegExp(parser)) {
          return parser.exec(value);
        }
        throw new TypeError('parser must be boolean|regexp|function');
      }
    }
  }
  has(header, matcher) {