Fix HttpLexer to allow HTTP/2.0 as preface

[rugk]

According to RFC7540 (section 3.5), if I interpret it correctly, it actually even has to start with 2.0 and not 2.

As HTTP/3 is also defined there, I have not looked into how that has to be used/started. (i.e. that may need to be fixed as well)

Fixes #1520

[ocket8888]

That part of the spec is talking about TCP frames to be sent to establish an HTTP/2 connection, not the request line of an HTTP/2 request. I think e.g. GET / HTTP/2.0 might be acceptable, but it's certainly doesn't have to end with .0, and curl for example definitely omits that part:

$ curl --http2-prior-knowledge -skv github.com
* Rebuilt URL to: github.com/
*   Trying 140.82.113.4...
* TCP_NODELAY set
* Connected to github.com (140.82.113.4) port 80 (#0)
* Using HTTP2, server supports multi-use
* Connection state changed (HTTP/2 confirmed)
* Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0
* Using Stream ID: 1 (easy handle 0x7fffd48b8860)
> GET / HTTP/2
> Host: github.com
> User-Agent: curl/7.58.0
> Accept: */*
>
...

[birkenfeld]

So I've looked at the RFC, and there are several things to note here.

Most important is that HTTP 2 doesn't even have a request header line or status line anymore (see section 8.1.2.1). What is sent after the HTTP/2 negotiation (the PRI * line from section 3.5) is mostly binary anyway. What curl shows here is not what is going on the wire, but a representation/"translation" of this HTTP/2 conversation to a HTTP1-like text-only version.

Still it makes sense to accept this in the lexer, since something like this is the most likely representation of a HTTP/2 session used as its input...

[rugk]

What is sent after the HTTP/2 negotiation (the PRI * line from section 3.5) is mostly binary anyway

Ah, yeah, so actually that does include the string HTTP/2.0. But well yeah, I guess for interpretation/display purposes, we just should not be so strict and accept requests that look like a proper HTTP request.

[rugk]

Note: Remember to check whether HTTP/3 may need the same addition.