Fix detecting os and platform for newer Gecko

It fails detecting OS and platform for Gecko UA string like this "Mozilla/5.0 (Windows NT 6.1; rv:17.0) Gecko/20100101 Firefox/17.0", it returns "rv:17.0" for OS, so I tried to fix it according [Gecko user agent reference](https://developer.mozilla.org/en-US/docs/Gecko_user_agent_string_reference).

Unit tests included.