Seems like customContextFile, customClientContextFile options doesn't work

I've tried provide custom files, but no success. I copied [client.html](https://github.com/karma-runner/karma/blob/master/static/client.html) to my project root and provided config like that
```
module.exports = function (config) {
config.set({
...
customClientContextFile: 'client.html'
});
};
```
but it get's file from lib. I tried to do some debug, logs in config.js before return shows it gets path correct initialy
https://github.com/karma-runner/karma/blob/50f963551fbccf669c8d42499d0406ded2ec6176/lib/config.js#L269

but then here it uses lib default file
https://github.com/karma-runner/karma/blob/50f963551fbccf669c8d42499d0406ded2ec6176/lib/middleware/karma.js#L75
Maybe I did something wrong?