### 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
if (platform.isStandardBrowserEnv) {
      
      
      const xsrfValue = isURLSameOrigin(fullPath) && config.xsrfCookieName && cookies.read(config.xsrfCookieName);
      if (xsrfValue) {
        requestHeaders.set(config.xsrfHeaderName, xsrfValue);
      }
    }
    
    requestData === undefined && requestHeaders.setContentType(null);
    
    if ('setRequestHeader' in request) {
      utils.forEach(requestHeaders.toJSON(), function setRequestHeader(val, key) {
        request.setRequestHeader(key, val);
      });
    }
    
    if (!utils.isUndefined(config.withCredentials)) {
      request.withCredentials = !!config.withCredentials;
    }
    
    if (responseType && responseType !== 'json') {
      request.responseType = config.responseType;
    }
    
    if (typeof config.onDownloadProgress === 'function') {
      request.addEventListener('progress', progressEventReducer(config.onDownloadProgress, true));
    }