Feature Request: Disable X-Forwarded-For headers

### Expected behaviour
I should be able to disable the X-Forwarded-For headers.

### Actual behaviour
Right now it's not possible to disable the X-Forwarded-For headers supplied to httpProxy.createProxyServer

https://github.com/karma-runner/karma/blob/f13f4e4f744dd918204dc2eb9678b15344943068/lib/middleware/proxy.js#L53-L63

I'm talking about the xfwd field.

### Environment Details
- Karma version (output of `karma --version`):
Karma version: 1.7.1

- Relevant part of your `karma.config.js` file
```
proxies: {
"/assets/": "/base/src/assets/",
"/services/": {
target: "https://domain.com/services/",
changeOrigin: true
}
},
```