parse::<Uri> fails to parse uris with triple slash after scheme

URIs that reference resources on a local filesystem often use URIs of the format <scheme>://<path_to_file>. If the path is absolute that leads to URIs with 3 slashes after the scheme like unix:///var/run/socket.sock

The following expression panics with InvalidUri(InvalidFormat):

"unix:///path/to/foo.sock".parse::<Uri>().unwrap();

while a similar expression using a relative file path like unix://relative/path/to.sock works just fine.

While I understand that the http crate does not aim to provide a completely compliant URI parser I still think this should be fixed here because:

• There are http services that do listen on unix sockets (docker for example)
• Allowing relative file and unix URIs while not allowing absolute ones is inconsistent

Start at the Uri parser entry point exercised by "unix:///path/to/foo.sock". Reproduce the InvalidUri(InvalidFormat) panic, compare it with the working relative-path form, and verify that absolute triple-slash unix or file-style URIs parse without breaking existing relative URI behavior.