Calls to dbus hang when using invalid characters

[bcwaldon]

Repro steps

1. Create a new instance of go-systemd's dbus.Conn
2. Call GetUnitInfo with a dbus path containing unsupported characters

Result:
The process will hang

Expected:
Return an error telling me I've asked to do the impossible

Here's a script to illustrate the problem:

package main

import (
        systemdDbus "github.com/coreos/go-systemd/dbus"
        "github.com/guelfey/go.dbus"
        "os"
)

func main() {
        systemd := systemdDbus.New()
        path := dbus.ObjectPath(os.Args[1])
        info, _ := systemd.GetUnitInfo(path)
        println(info["ActiveState"].Value().(string))
}

Build this and run something like

./test ./test /org/freedesktop/systemd1/unit/ping.service

vs.

./test /org/freedesktop/systemd1/unit/ping_2eservice

This describes valid dbus paths: http://dbus.freedesktop.org/doc/dbus-specification.html#message-protocol-marshaling-object-path

This should illustrate what encoding systemd expects: http://cgit.freedesktop.org/systemd/systemd/tree/src/shared/util.c#n1360

[philips]

Fixed via #25. Thanks again @jqln-0