how query volume through metadata

code:

```
	vmeta := map[string]string{ <br>
		"pod_id": "68d32002-8d53-11e7-8780-48df370580b8",
	}
	listOpts := volumes.ListOpts{AllTenants: true, Metadata: vmeta}
	query, err := listOpts.ToVolumeListQuery()
	if err != nil {
		panic(err)
	}
	fmt.Println(query)
```
go run got:
```
?all_tenants=true
```

why missing metadata? how to query volume with metadata