URL validation makes it impossible to implement unix domain socket transport

[algesten]

The URL validation in ureq3 is too strict. Even if ureq3 doesn't claim to support unix domain sockets itself, it's not possible to implement a transport that could do it.

This is stricter than what ureq3 is intended to be.

I just spent a day trying to get this work, and couldn't. There's no way I could find to avoid the check in default_port:

ureq/src/util.rs

Lines 49 to 59 in 1ce334d

	fn default_port(&self) -> Option<u16> {
	if *self == Scheme::HTTPS {
	Some(443)
	} else if *self == Scheme::HTTP {
	Some(80)
	} else if let Ok(proxy) = ProxyProtocol::try_from(self.as_str()) {
	Some(proxy.default_port())
	} else {
	debug!("Unknown scheme: {}", self);
	None
	}

Which always results in a parsing error:

ureq/src/util.rs

Lines 319 to 321 in 1ce334d

	scheme
	.default_port()
	.ok_or_else(|| Error::BadUri(format!("unknown scheme: {}", scheme)))?;

You could just pass a completely fake uri to the agent, I guess, but that's pretty ugly. I think this issue should be reopened.

Originally posted by @colinmarc in #538

[algesten]

The validation should probably be up to the transport chain. Some refactoring to make that happen.

[algesten]

This is somewhere between a bug and enhancement. The intention is to support the use case, but the current behavior isn't exactly broken since it's deliberate.