[feature] the request part should be one single string

[hsluoyz]

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"

[hsluoyz]

@tangyang9464 @imp2002