Change suggestion on loadPolicyLine

[mariohov]

@hsluoyz

After a changed on LoadFilteredPolicy Enforcer's function to keep model data, I suggest altering loadPolicyLine to only append new Rules if it's not yet loaded into the model.

Example:

func loadPolicyLine(line CasbinRule, model model.Model) {
...
...
LineEnd:
	if !model.HasPolicy(sec, line.PType, tokens) {
		model[sec][key].Policy = append(model[sec][key].Policy, tokens)
	}
}