### Describe the issue

I set `axios.defaults.headers.common['Content-Type'] = 'application/json'`. When firing a `axios.patch` request (without a config), the request uses `application/x-www-form-urlencoded` instead.

### Example Code

```js
// This does not work as expected:
axios.defaults.headers.common['Content-Type'] = 'application/json'

// This does:
axios.defaults.headers.common['Content-Type'] = 'application/json'
axios.defaults.headers.patch['Content-Type'] = 'application/json'
```


### Expected behavior

I expected POST requests to use `axios.defaults.headers.common`, too.

I think this is where the problem (or intended behaviour) comes from: https://github.com/axios/axios/blob/3f53eb6960f05a1f88409c4b731a40de595cb825/lib/core/dispatchRequest.js#L46

### Axios Version

1.4.0

### Adapter Version

_No response_

### Browser

Firefox

### Browser Version

116.0.3 (64-bit)

### Node.js Version

_No response_

### OS

_No response_

### Additional Library Versions

_No response_

### Additional context/Screenshots

_No response_
test/specs/headers.spec.js
'Content-Type': null,
        'x-header-a': null,
        'x-header-b': undefined
      }
    }).catch(function (err) {
      done(err);
    });
    getAjaxRequest().then(function (request) {
      testHeaderValue(request.requestHeaders, 'Content-Type', undefined);
      testHeaderValue(request.requestHeaders, 'x-header-a', undefined);
      testHeaderValue(request.requestHeaders, 'x-header-b', undefined);
      testHeaderValue(request.requestHeaders, 'x-header-c', 'c');
      done();
    }).catch(done);
  });
  it('should use application/json when posting an object', function (done) {
    axios.post('/foo/bar', {
      firstName: 'foo',
      lastName: 'bar'
    });
    getAjaxRequest().then(function (request) {
      testHeaderValue(request.requestHeaders, 'Content-Type', 'application/json');
      done();
    });
  });
  it('should remove content-type if data is empty', function (done) {
    axios.post('/foo');
    getAjaxRequest().then(function (request) {
      testHeaderValue(request.requestHeaders, 'Content-Type', undefined);
      done();