#633 uri::split fails when username contains backslash

[Pat Thoyts]

For Windows NT domains a users ID is typically composed
of their NT domain joined with the login name. So we
typically end up setting our http_proxy environment
variable to something like:
http_proxy='http://DOMAIN\loginid@webproxy:80/'

Unfortunately uri::split makes a meal of this and puts
everything after the backslash into the path element.

Needed for the autoproxy package.