[minor test] Add tests for File store `save()`. Improve default file format to pretty print the JSON output

.gitignore

@@ -1,3 +1,3 @@
 .DS_Store
 config.json
-test/fixtures/store.json
+test/fixtures/*.json

lib/nconf/stores/file.js

@@ -24,7 +24,12 @@ var File = exports.File = function (options) {
 
   this.type   = 'file';
   this.file   = options.file;
-  this.format = options.format || JSON;
+  this.format = options.format || {
+    stringify: function (obj) {
+      return JSON.stringify(obj, null, 2)
+    },
+    parse: JSON.parse
+  };
 };
 
 // Inherit from the Memory store

test/file-store-test.js

@@ -19,7 +19,7 @@ vows.describe('nconf/stores/file').addBatch({
   "When using the nconf file store": {
     topic: function () {
       var filePath = path.join(__dirname, 'fixtures', 'store.json');
-      fs.writeFileSync(filePath, JSON.stringify(data));
+      fs.writeFileSync(filePath, JSON.stringify(data, null, 2));
       store = new nconf.stores.File({ file: filePath });
       return null;
     },
@@ -33,6 +33,33 @@ vows.describe('nconf/stores/file').addBatch({
       }
     }
   }
+}).addBatch({
+  "When using the nconf file store": {
+    topic: function () {
+      var tmpPath = path.join(__dirname, 'fixtures', 'tmp.json'),
+          tmpStore = new nconf.stores.File({ file: tmpPath });
+      return tmpStore
+    },
+    "the save() method": {
+      topic: function (tmpStore) {
+        var that = this;
+        
+        Object.keys(store.store).forEach(function (key) {
+          tmpStore.set(key, store.store[key]);
+        });
+        
+        tmpStore.save(function () {
+          fs.readFile(tmpStore.file, function (err, data) {
+            return err ? that.callback(err) : that.callback(err, JSON.parse(data.toString()));
+          });
+        });
+      },
+      "should save the data correctly": function (err, data) {
+        assert.isNull(err);
+        assert.deepEqual(data, store.store);
+      }
+    }
+  }
 }).addBatch({
   "When using the nconf file store": {
     "the set() method": {