subset url parameter appears to be broken

When selecting an additional language through GF the _subset_ query parameter is added to the embed code. The problem is this doesn't appear to change the returned css in any way.

**_Example:_**
Generated by GF:
https://fonts.googleapis.com/css?family=Roboto+Condensed&subset=cyrillic-ext,vietnamese
Unescaped version:
https://fonts.googleapis.com/css?family=Roboto+Condensed&subset=cyrillic-ext,vietnamese
Without subset:
https://fonts.googleapis.com/css?family=Roboto+Condensed

All URLs return the exact same CSS when requested on Chrome 68 on Windows 10. More specifically they all return all the available languages of the font. The unfortunate result is a css-file that is often 5, 6 or 7 times as large as it has to be when the developer is certain that only _latin_ is needed. While I get that only the actual fonts that are used are fetched (using the unicode range etc.) this 'useless' CSS still doesn't feel right in a _perf matters_ world