Add support for range requests to the server

[zhuker]

Expected behavior

Files served by karma http server should support Range: bytes=42-43

Actual behavior

Entire file is output

Enviroment Details

Karma version: 0.13.22

• Karma version (output of karma --version):
• Relevant part of your karma.config.js file

Steps to reproduce the behaviour

1. Write in a test

var xhr = new XMLHttpRequest();
xhr.open("GET", "/base/big_binary_file.dat", true);
xhr.setRequestHeader("Range", "bytes=42-43");
xhr.responseType =  "arraybuffer";
xhr.onload = () => {
expect(xhr.response.byteLength).toBe(2) 
}

1. Run test
2. Test fails

[dignifiedquire]

Thanks for the report, it's an interesting feature so if someone wants to implement it I'd be happy to accept a PR for it.