How could I set multiple `Access-Control-Allow-Origin` headers with Rack compatitable application.

Specification said that I should return `[status, headers, body]` array as a result. Headers is a hash of headers :). So I can't set same header twice.

```
headers = {}
headers["Access-Control-Allow-Origin"] = "http://my.domain1.com"
headers["Access-Control-Allow-Origin"] = "http://my.domain2.com"

```

won't ever work.

What should I do in my case? How can I send two identical headers?