Poco::URI: don't lowercase host part if it's a Unix domain socket

[obiltschnig]

Some (non-standard) URI schemes for Unix domain sockets use the host part to specify the path of a Unix domain socket, e.g. http+unix://%2Ftmp%2FServerSocket/index.html. In such a case, the URI class should not change (convert to lower case) the host part, as file names are case sensitive on some platforms.

So if _host starts with a %, don't call toLowerInPlace() in URI::parseHostAndPort.