Avoid to pass an undefined args argument to spawn (#34)

* Avoid to pass an undefined args argument to spawn as it makes the option object being ignored
fix #33

* comply to code style

lib/git-hooks.js

@@ -185,6 +185,7 @@ function spawnHook(hookName, args) {
     if (!isHookExecutable) {
         throw new Error('Cannot execute hook: ' + hookName + '. Please check file permissions.');
     }
+    args = args || [];
     return spawn(hookName, args, {stdio: 'inherit'});
 }