Push: RefSpec deleting reference does not work

Example:
``` go
r, err := git.PlainOpen(path)
	CheckIfError(err)

	Info("git push")
	err = r.Push(&git.PushOptions{
		RefSpecs: []config.RefSpec{
			config.RefSpec(":master"),
		},
	})
	CheckIfError(err)
```
Obtained error:
```
error: already up-to-date
```
Expected message:
```
$ git push origin :master
To file://path/to/remote/go-git-remote
 - [deleted]         master
```