[feature] the request part should be one single string

See:

```
./casbin enforce -m "examples/rbac_model.conf" -p "examples/rbac_policy.csv" "alice" "data1" "read"
```

It should be:

```
./casbin enforce -m "examples/rbac_model.conf" -p "examples/rbac_policy.csv" -r "alice, data1, read"
```