func (a *Args) Add(key, value string) {
	a.args = appendArg(a.args, key, value, argsHasValue)
}
func (a *Args) Add(key, value string) {
	a.args = appendArg(a.args, key, value, argsHasValue)
}
// Add adds 'key=value' argument.
//
// Multiple values for the same key may be added.