### Describe the bug

After upgrading msw from 1.x to 2.x `FormData` upload stopped working in tests. Looking at the debugging output, Axios doesn't append the `boundary` to the `Content-Type` header anymore.

The MSW team checked [1] this already on their side.

I tried to append the boundary manually to the header for debugging purposes, but that revealed another issue:
```
{ "Content-Type": "multipart/form-data; boundary=abc" }
```
ends in
```
Content-Type: multipart/form-databoundary=abc
```


[1] https://github.com/mswjs/msw/issues/1835


### To Reproduce

Checkout the msw reproducer at [1] and
```
cd examples/with-vitest
npm install
npm test
```


[1] https://github.com/pschyma/msw-examples/tree/axios-formdata-reproducer

### Code snippet

_No response_

### Expected behavior

As with msw the `boundary` is appended to `Content-Type` header.

### Axios Version

1.6.0

### Adapter Version

XHR

### Browser

jsdom

### Browser Version

_No response_

### Node.js Version

18.8.2

### OS

Linux

### Additional Library Versions

```bash
msw 2.0.x
vitest 0.34.6
```


### Additional context/Screenshots

_No response_
lib/adapters/xhr.js
reject(err);
        done();
      }, response);
      
      request = null;
    }
    if ('onloadend' in request) {
      
      request.onloadend = onloadend;
    } else {
      
      request.onreadystatechange = function handleLoad() {
        if (!request || request.readyState !== 4) {
          return;
        }
        
        
        
        
        if (request.status === 0 && !(request.responseURL && request.responseURL.indexOf('file:') === 0)) {
          return;
        }
        
        
        setTimeout(onloadend);
      };
    }
    
    request.onabort = function handleAbort() {
      if (!request) {