Fixed msgid_plural error

lib/po.js

@@ -66,6 +66,7 @@ PO.parse = function(data) {
     'POT-Creation-Date': '',
     'PO-Revision-Date': '',
     'Last-Translator': '',
+    'Language': '',
     'Language-Team': '',
     'Content-Type': '',
     'Content-Transfer-Encoding': '',
@@ -152,7 +153,7 @@ PO.parse = function(data) {
 PO.Item = function() {
   this.msgid = '';
   this.references = [];
-  this.msgid_plural = '';
+  this.msgid_plural = null;
   this.msgstr = [];
   this.comments = [];
 };