Unsupported HTTP versions should return 505 HTTP Version Not Supported

Requests made with unsupported HTTP versions (HTTP/2.0) are accepted and processed as if they were HTTP 1.1. They should get 505 HTTP Version Not Supported responses instead. If HTTP 2.0 actually happens some day, there will be an issue with older snap servers claiming to support it, but not actually supporting it.

[hvr]

Btw, RFC 2145 (Use and Interpretation of HTTP Version Numbers) is relevant to this issue, as it describes in which cases a HTTP server is supposed to accept client requests with higher HTTP versions w.r.t. the version the HTTP server implements.

[gregorycollins]

So looks like we should only respond with a 505 if the HTTP major version is >= 2.

[hvr]

maybe :-)

...but fwiw, when you send a faked GET / HTTP/2.0/Host: foobar request to Apache 2.2 (which is without doubt one of the mainstream web-server deployed), it happily responds with a HTTP/1.1 200 OK...

and I assume that HTTP 2.0 client will have to take this into account anyway; i.e. fallback to HTTP 1.1 if the server responds with a HTTP/1.1 response...

Oh, apache does flub this too. I appear to have forgotten to send a Host header when I tried it on apache before submitting this. Given that apache does it, that pretty much rules out any future practical concerns, since apache is too big to ignore.