YAML: Serialize nodes properly

This fixes the error where a YamlList is not of type string.

lib/storage/serializers.dart

@@ -86,7 +86,8 @@ class MarkdownYAMLSerializer implements NoteSerializer {
     var str = "";
 
     map.forEach((key, value) {
-      str += key + ": " + value + "\n";
+      String val = value.toString();
+      str += key + ": " + val + "\n";
     });
     return str;
   }

test/serializers_test.dart

@@ -57,6 +57,20 @@ created: 2017-02-15T22:41:19+01:00
 foo: bar
 ---
 
+Alright.""";
+
+      var serializer = MarkdownYAMLSerializer();
+      var note = serializer.decode(str);
+      var actualStr = serializer.encode(note);
+
+      expect(actualStr, str);
+    });
+
+    test('Markdown Serializer YAML Lists', () {
+      var str = """---
+foo: [bar, gar]
+---
+
 Alright.""";
 
       var serializer = MarkdownYAMLSerializer();