HTTP/1 and HTTP/2

Starting v0.3.1, a specific version of HTTP can be configured. By default, if Volga dependency added like this:

[dependencies]
volga = "0.3.1"

There HTTP/1 will be used, and if we need HTTP/2, it can be enabled if we explicitly add the http2 feature or use the full.

[dependencies]
volga = { version = "0.3.1", features = ["full"] }

With the full the HTTP/2 will be used if it is possible, otherwise it will be switched to HTTP/1 automatically.