卷曲：单线测试http/2支持

我目前正在编写单元测试，以检查是否http/2受支持。

是否有一个curl单线可检查是否http/2受支持并输出易于解析的响应？

HTTP/2 supported:

$ curl -sI https://curl.haxx.se -o/dev/null -w '%{http_version}\n'
2

HTTP/2 not supported (instead serving 1.1 in this case):

$ curl -sI http://curl.haxx.se -o/dev/null -w '%{http_version}\n'
1.1

(curl 7.50.0 or later is required for this command line to work)