HTTP::Client: check host is just a host

[asterite]

Fixes #7902

To be honest, I'd really like to change HTTP::Client to work like in other languages (Go, C#), like some of what is proposed in #6011

But until that happens I think it's good if we make it easier to use the existing HTTP::Client.

[bcardiff]

I guess the def self.new(uri : URI, tls = nil) should also have a runtime check that the URI does not contains a path.

The protected method validate_host seems to be used in other places to ensure there is a host, but making it validate that the URI has no path will break some stuff probably.

[asterite]

@bcardiff Yeah, I saw that too but I think that's a different issue to fix (I just wanted to fix one thing, the other one isn't reported so it might not be a problem, but we'd still want to do it). Plus maybe when you create an HTTP::Client with a path it makes sense to use that as a base path, even merging the query string. I don't know...